Update Contributing

Author: zspitz

CONTRIBUTING.md

@@ -1 +1,15 @@
-TODO
+# Contributing
+
+Thank you for your interest in contributing to this project! There are a number of ways you can do so:
+
+* **Testing and bug-reporting** -- This project is currently only being developed by one person, and it is very likely that various issues will slip through the cracks. Use the rendering library in your own projects, test the visualizer while developing, and report issues here on GitHub.
+* **React or comment on [open issues](https://github.com/zspitz/ExpressionToString/issues)**
+* **Contributing code** -- There are three major areas of interest:
+
+  * Formatter extensibility -- [#54](https://github.com/zspitz/ExpressionToString/issues/54) and [#38](https://github.com/zspitz/ExpressionToString/issues/38)
+  * [Visual Studio integration](https://github.com/zspitz/ExpressionToString/issues?q=is%3Aissue+is%3Aopen+label%3A%22vs+integration%22)
+  * [AvalonEdit](https://github.com/zspitz/ExpressionToString/issues/73) for multiple selection in the source code
+
+  See the [Solution and projects structure](https://github.com/zspitz/ExpressionToString/wiki/Solution-structure) wiki page, to understand what code goes where.
+
+* **Upvoting [external issues](https://github.com/zspitz/ExpressionToString/wiki/External-issues)** -- While working on this project, I've come across a number of limitations in external APIs, and have filed corresponding bugs.