add 'how to contribute' in CONTRIBUTING.md

Author: bjornregnell

CONTRIBUTING.md

@@ -0,0 +1,17 @@
+# Contributing to `introprog-scalalib`
+
+Contributions are welcome!
+
+1. Open an **issue** and start discussing your suggestions. Issues can comprise e.g. a bug, a documentation improvement, a new or enhanced feature, beginner-friendly examples, tests, etc.
+
+2. **Fork** this repo.
+
+3. Do your changes and additions in line with the issue discussions. Make minimal, coherent commits with good commit messages.
+
+4. When you are ready with an enhancement that compiles and run according to expectations, create a **pull request** (PR). Make sure your fork is up-to-date with the master repo before creating a PR.
+
+Please keep the general intention of this repo in mind as outlined in the README; in summary: *Beginner-friendly API:s usable with simple Scala*
+
+Further help on how to keep it simple:
+
+*  http://www.lihaoyi.com/post/StrategicScalaStylePrincipleofLeastPower.html