Re-add README

Author: JakubVanek

README.md

@@ -0,0 +1,58 @@
+# OpenJDK for EV3
+A custom build of OpenJDK 9 & 10 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
+* EV3 jmods
+  * It can be used for creating custom JRE images.
+
+## Building
+
+0. Clone/download this repo to your computer.
+1. Install [Docker](https://docs.docker.com/engine/installation/) for your operating system.
+2. Adjust the JAVA_VERSION variable in `jdkX/config.sh` to match the current OpenJDK tip:
+  * [OpenJDK 9 updates](http://hg.openjdk.java.net/jdk-updates/jdk9u/) repo
+  * [OpenJDK 10 updates](http://hg.openjdk.java.net/jdk-updates/jdk10u/) repo
+3. Build the jdk cross-compilation environment:
+  * For JDK9:
+```sh
+sudo docker build -t ev3dev-lang-java:jdk9-stretch -f jdk9/Dockerfile.system  build
+sudo docker build -t ev3dev-lang-java:jdk9-build   -f jdk9/Dockerfile.scripts build
+```
+  * For JDK10:
+```sh
+sudo docker build -t ev3dev-lang-java:jdk10-stretch -f jdk10/Dockerfile.system  build
+sudo docker build -t ev3dev-lang-java:jdk10-build   -f jdk10/Dockerfile.scripts build
+```
+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).
+  * For JDK9:
+```
+sudo docker run --rm -it -v <path on host, where the sources should be stored>:/build ev3dev-lang-java:jdk9-build
+```
+  * For JDK10:
+```
+sudo docker run --rm -it -v <path on host, where the sources should be stored>:/build ev3dev-lang-java:jdk10-build
+```
+5. Let's fetch the OpenJDK sources:
+```
+./fetch.sh
+```
+6. The OpenJDK source tree should be ready. Now you can start the cross-build itself:
+```
+./build.sh
+```
+7. Create the zipped images:
+```
+./zip.sh
+```
+8. If the build was successful, JDK packages were created in `/build/jre-ev3.tar.gz`, `/build/jdk-ev3.tar.gz` and `/build/jmods.tar.gz`.
+If you have mounted `/build`, you can access the files from the host.
+
+## ~~JShell on the EV3~~
+
+No longer supported. JShell backend on the EV3 is a huge overkill.