Update README to include build instructions

Author: nhaarman

README.md

@@ -34,3 +34,34 @@ fun doAction_doesSomething(){
 ```
 
 For more info and samples, see the [Wiki](https://github.com/nhaarman/mockito-kotlin/wiki).
+
+## Building
+
+Mockito-Kotlin is built with Gradle.
+
+ - `./gradlew build` builds the project
+ - `./gradlew publishToMavenLocal` installs the maven artifacts in your local repository
+ - `./gradlew assemble && ./gradlew test` runs the test suite (See Testing below)
+
+### Versioning
+
+Mockito-Kotlin roughly follows SEMVER; version names are parsed from 
+git tags using `git describe`.
+
+### Testing
+
+Mockito-Kotlin's test suite is located in a separate `tests` module,
+to allow running the tests using several Kotlin versions whilst still
+keeping the base module at a recent version.  
+
+Testing thus must be done in two stages: one to build the base artifact
+to test against, and the actual execution of the tests against the 
+built artifact:
+
+ - `./gradlew assemble` builds the base artifact
+ - `./gradlew test` runs the tests against the built artifact.
+
+Usually it is enough to test only using the default Kotlin versions; 
+CI will test against multiple versions.
+If you want to test using a different Kotlin version locally, set
+an environment variable `KOTLIN_VERSION` and run the tests.