added docs for core features (#93)

Author: AlexGalichenko

docs/intro.md

@@ -47,5 +47,179 @@ If you have the basic config `npx qavajs` will launch test execution.
 To specify custom path to the config file use `npx qavajs run --config <config>`.  
 In case if your config exports an object with multiple properties, you can specify which property to read `npx qavajs run --profile <profile>`.
 
+### 🛠️ Override memory values
+In case if tests need to be run with updated memory value they can be passed via CLI (e.g run scenarios on some other url)
+It can be done by passing `--memory-values` parameter which is JSON with params that need to be overridden. 
+For instance, to override `$url` value:
+
+```bash
+npx qavajs run --config config.ts --memory-values '{"url": "https://github.com"}'
+```
+
+### 🛠️ Pass CLI params to workers
+All params that you passed to qavajs cli will be available in `CLI_ARGV` environment variable in all child workers.
+
+### 🛠️ Memory value parameter type
+`value` parameter type provides API to access memory
+
+```typescript
+When('Read memory {value}', async function(memoryValue) {
+    expect(memoryValue.value()).to.equal('ts');
+});
+
+When('Set memory {value} as {string}', async function(memoryKey, value) {
+    memoryKey.set(value);
+});
+```
+
+### 🛠️ Validation parameter type
+`validation` parameter type provides API to verify values by certain condition
+
+```typescript
+When('I expect {string} {validation} {string}', async function(value1, validate, value2) {
+    validate(value1, value2);
+});
+```
+
+### 🛠️ Test Sharding
+qavajs provides ability to shard your tests between different machines. To do so pass `--shard x/y` parameter in CLI,
+where x - current shard, y - total number of shards.
+
+```
+npx qavajs run --config config.js --shard 1/2
+npx qavajs run --config config.js --shard 2/2
+```
+
+### 🛠️ Execute steps from other steps
+It is possible to implement complex logic using built-in qavajs steps via `executeStep` world method
+```typescript
+When('I do smth complex', async function() {
+    await this.executeStep(`I type 'username' to 'Username Input'`);
+    await this.executeStep(`I type 'password' to 'Password Input'`);
+    await this.executeStep(`I click 'Login Button'`);
+    await this.executeStep(`I fill following fields`, new DataTable([
+        [ 'Order', '123' ],
+        [ 'Delivery Location', 'New York' ]
+    ]))
+});
+```
+
+### 🛠️ World
+Module extends CucumberJS world with additional entities
+
+| entity      | type     | description                                      | example                                                            |
+|-------------|----------|--------------------------------------------------|--------------------------------------------------------------------|
+| config      | object   | loaded config                                    | `this.config.parallel`                                             |
+| executeStep | function | programmatically execute certain step definition | `await this.executeStep("I type 'username' to 'Username Input'");` |
+| setValue    | function | set memory value                                 | `await this.setValue('key', 'value');`                             |
+| getValue    | function | get memory value or expression                   | `await this.getValue('$key');`                                     |
+| validation  | function | get validation function based                    | `await this.getValue('to equal');`                                 |
+
+### 🛠️ Override step definition
+`Override` function provides capability to override step implementation and avoid ambiguous exception
+
+```typescript
+import { Override } from '@qavajs/core';
+
+When('I do test', async function () {});
+
+Override('I do test', async function () {
+    console.log('I am overridden');
+});
+```
+
+### 🛠️ Fixture
+`Fixture` provides convenient way to prepare test environment for specific test.
+
+This example will open pdp page before test and clean cart after test
+```typescript
+import { Fixture } from '@qavajs/core';
+
+Fixture('pdp', async function() {
+    await this.playwright.page.goto('https://my-site/pdp');
+    // fixture may return function that will be executed after test
+    return async function() {
+        await this.playwright.page.request.get('/cleanCart');
+    }
+});
+```
+
+```gherkin
+Feature: feature with fixture
+
+  @pdp
+  Scenario: scenario with fixture
+    When I click 'qavajs T-shirt'
+    And I click 'cart icon'
+    Then I expect 'qavajs T-shirt cart item' to be visible
+```
+
+### 🛠️ Template
+`Template` provides a way to define step definition using Gherkin language
+
+```typescript
+import { When, Template } from '@qavajs/core';
+
+When('I click {string} and verify {string}', Template((locator, expected) => `
+    I click '${locator}'
+    I expect '${locator} > Value' to equal '${expected}'
+`));
+```
+
+### 🛠️ Test Execution Hooks
+`BeforeExecution` and `AfterExecution` allow to define hooks that will be executed
+once before/after whole test execution
+
+```typescript
+import { BeforeExecution, AfterExecution } from '@qavajs/core';
+import { Server } from './server';
+
+const server = new Server();
+
+BeforeExecution(async function () {
+    await server.start();
+});
+
+AfterExecution(async function () {
+    await server.stop();
+});
+```
+
+### 🛠️ Service
+Services is an entities that can execute logic before and after whole test run.
+
+```typescript
+import externalService from './externalService';
+
+export default {
+    service: [
+        {
+            options: {
+                data: 42
+            },
+            before() {
+                console.log(this.options.data);
+            },
+            after(result) {
+                if (!result.success) process.exitCode = 1;
+            }
+        },
+        {
+            options: {
+                data: 42
+            },
+            ...externalService
+        }
+    ]
+}
+```
+There is a one minute-long default timeout for a before and after test logic to prevent entire process from freezing.
+To set up a custom timeout in milliseconds use serviceTimeout property in the config file
+```typescript
+export default {
+    serviceTimeout: 1_200_000
+}
+```
+
 ### 📘 Extra
 [Code Examples](https://github.com/qavajs/demo)