Update README.md (#498)

Update README.md with build instructions etc. Roughly a copy of ESH README.md, but a good first start, I guess.

Signed-off-by: davidgraeff <david.graeff@web.de>

README.md

@@ -1,2 +1,44 @@
-This project contains some core bundles of the openHAB 2 runtime.
-They are picked up by the main [openHAB distribution build](https://github.com/openhab/openhab-distro).
+# openHAB 2 Core bundles
+
+This project contains core bundles of the openHAB 2 runtime.
+
+Building and running the project is fairly easy if you follow the steps detailed below.
+
+Please note that openHAB 2 Core is not a product itself, but a framework to build solutions on top.
+It is picked up by the main [openHAB distribution build](https://github.com/openhab/openhab-distro).
+
+This means that what you build is primarily an artifact repository of OSGi bundles that can be used within smart home products.
+
+1\. Prerequisites
+=================
+
+The build infrastructure is based on Maven. 
+If you know Maven already then there won't be any surprises for you. 
+If you have not worked with Maven yet, just follow the instructions and everything will miraculously work ;-)
+
+What you need before you start:
+- Maven3 from https://maven.apache.org/download.html
+
+Make sure that the "mvn" command is available on your path
+
+2\. Checkout
+============
+
+Checkout the source code from GitHub, e.g. by running
+
+git clone https://github.com/openhab/openhab-core.git
+
+3\. Building with Maven
+=======================
+
+To build this project from the sources, Maven takes care of everything:
+- set MAVEN_OPTS to "-Xms512m -Xmx1024m"
+- change into the openhab-core directory ("cd openhab-core“)
+- run "mvn clean install" to compile and package all sources
+
+If there are tests that are failing occasionally on your local build, run `mvn -DskipTests=true clean install` instead to skip them.
+
+# How to contribute
+
+If you want to become a contributor to the project, please read about [contributing](https://www.openhab.org/docs/developer/contributing/contributing.html) and check our [guidelines](https://www.openhab.org/docs/developer/development/guidelines.html) first.
+