Improve documentation for contributors/maintainers

It was pointed out recently that the link to the Contributing guide is
buried at the bottom of the README. This commit makes it more prominent
by moving it to the top and lists the major sections of the document.

However, this commit also updates the Contributing guide to be more
generally helpful and to answer questions that I've received from other
teams not familiar with this repo who are unaware that codeownership and
releases are shared among teams.

Author: mcmire

README.md

@@ -2,9 +2,21 @@
 
 This monorepo is a collection of packages used across multiple MetaMask clients (e.g. [`metamask-extension`](https://github.com/MetaMask/metamask-extension/), [`metamask-mobile`](https://github.com/MetaMask/metamask-mobile/)).
 
-## Modules
+## Contributing
 
-This repository contains the following packages [^fn1]:
+See the [Contributor Guide](./docs/contributing.md) for help on:
+
+- Setting up your development environment
+- Working with the monorepo
+- Testing changes in clients
+- Issuing new releases
+- Creating a new package
+
+## Installation/Usage
+
+Each package in this repository has its own README where you can find installation and usage instructions. See `packages/` for more.
+
+## Packages
 
 <!-- start package list -->
 
@@ -46,8 +58,6 @@ This repository contains the following packages [^fn1]:
 
 <!-- end package list -->
 
-Or, in graph form [^fn1]:
-
 <!-- start dependency graph -->
 
 ```mermaid
@@ -188,10 +198,4 @@ linkStyle default opacity:0.5
 
 <!-- end dependency graph -->
 
-Refer to individual packages for usage instructions.
-
-## Learn more
-
-For instructions on performing common development-related tasks, see [contributing to the monorepo](./docs/contributing.md).
-
-[^fn1]: The package list and dependency graph should be programmatically generated by running `yarn update-readme-content`.
+(This section may be regenerated at any time by running `yarn update-readme-content`.)