extended Readme.md

Signed-off-by: Stephan Richter <s.richter@srsoftware.de>

Readme.md

@@ -11,4 +11,10 @@ Currently, this project only depends on the following libraries:
 
 At the time of writing, these libraries have no further transitive dependencies, this mitigating any bloat from the project.
 
+## build
+
+This is a gradle project. To compile it, you should have a recent version of a Java Development Kit installed.
+
+Build the project by launching `./gradlew build` in a terminal while being in the root folder of the project.
+
 [specification]: https://openid.net/specs/openid-connect-core-1_0.html