[readme] Add component description

Author: JakubVanek

README.md

@@ -1,30 +1,45 @@
 # openjdk-9-ev3
-A custom Build of OpenJDK 9 Sources for EV3, a Lego Mindstorms Brick using ARM Soft Float
+A custom build of OpenJDK 9 for EV3, a Lego Mindstorms programmable brick featuring a ARM926EJ-S CPU.
+
+## Components
+The output consists of these parts:
+* JRE running on the EV3 - `jre-ev3.zip`
+  * Stripped down version -- a runtime image
+* JDK built for the EV3 - `jdk-ev3.zip`
+  * Basically the full build
+* [Official OpenJDK linux-amd64](http://jdk.java.net/9/) with EV3 jmods
+  * Only a stripped down version -- also a runtime image
+  * It can be used for creating custom JRE images. For this, jlink and the ev3 jmods are included.
+  * It has `javac` (jdk.compiler module).
+  * JShell remote launcher (hackish, linux-only) is available.
+
 
 ## Building
 
+0. Clone/download this repo to your computer.
 1. Install [Docker](https://docs.docker.com/engine/installation/) for your operating system.
-2. Build the jdk9 cross-compilation environment:
+2. Adjust the JAVA_VERSION variable in `build/config.sh` to match the current OpenJDK tip: [OpenJDK 9 updates](http://hg.openjdk.java.net/jdk-updates/jdk9u/)
+3. Build the jdk9 cross-compilation environment:
 ```sh
 sudo docker build -t ev3dev-lang-java:jdk9-system -f build/Dockerfile.system  build
 sudo docker build -t ev3dev-lang-java:jdk9-build  -f build/Dockerfile.scripts build
 ```
-3. Run the newly prepared container. You have to mount a host directory to the the `/build` directory in the container,
+4. Run the newly prepared container. You have to mount a host directory to the the `/build` directory in the container,
 otherwise the build would get discarded. The final build needs at least 6.5 GB of free space (in the build directory).
 ```
 sudo docker run --rm -it -v <path on host, where the sources should be stored>:/build ev3dev-lang-java:jdk9-build
 ```
-4. Let's fetch the OpenJDK sources:
+5. Let's fetch the OpenJDK sources:
 ```
 ./fetch.sh
 ```
-5. The OpenJDK source tree should be ready. Now you can start the cross-build itself:
+6. The OpenJDK source tree should be ready. Now you can start the cross-build itself:
 ```
 ./build.sh
 ```
-6. Create the zipped images:
+7. Create the zipped images:
 ```
 ./zip.sh
 ```
-7. If the build was successful, JDK9 packages were created in `/build/jre-ev3.zip` and `/build/jdk-ev3.zip`.
+8. If the build was successful, JDK9 packages were created in `/build/jre-ev3.zip`, `/build/jdk-ev3.zip` and `/build/jdk-pc.zip`.
 If you have mounted `/build`, you can access the files from the host.