Add build instructions

Author: JakubVanek

README.md

@@ -1,2 +1,27 @@
 # openjdk-9-ev3
 A custom Build of OpenJDK 9 Sources for EV3, a Lego Mindstorms Brick using ARM Soft Float
+
+## Building
+
+1. Install [Docker](https://docs.docker.com/engine/installation/) for your operating system.
+2. Build the jdk9 cross-compilation environment:
+```sh
+cd build
+sudo docker build -t jdk9cross .
+```
+3. Run the newly prepared container. We recommend mounting a host directory to the the `/build` directory in the container. At least 2 GB of free space will be needed.
+```
+sudo docker run -it -v <path on host, where the sources should be stored>:/build jdk9cross
+```
+4. Let's fetch the OpenJDK sources:
+```
+./fetch.sh
+```
+5. The OpenJDK source tree should be ready. Now you can start the cross-build itself:
+```
+./build.sh
+```
+6. If the build has successfully finished, JDK9 images are created in
+`/build/jdk9u/build/linux-arm-normal-client-release/images`.
+If you have mounted something to `/build`, you can also use
+`<path on host, where the sources should be stored>/jdk9u/build/linux-arm-normal-client-release/images`.