add initial contribution info

CONTRIBUTING.md

@@ -0,0 +1,23 @@
+Contributing to Leaflet
+=======================
+
+1. [Getting Involved](#getting-involved)
+2. [Reporting Bugs](#reporting-bugs)
+3. [Contributing Code](#contributing-code)
+4. [Improving Documentation](#improving-documentation)
+
+## Getting Involved
+
+Writing patches is not the only way to get involved with the development of Leaflet. You can help the project tremendously by discovering and reporting bugs, improving documentation, helping others on the [Leaflet forum](https://groups.google.com/forum/#!forum/leaflet-js) and [GitHub issues](https://github.com/CloudMade/Leaflet/issues), and spreading the word about Leaflet among your colleagues and friends.
+
+## Reporting Bugs
+
+TODO how to report bugs properly.
+
+## Contributing Code
+
+TODO building Leaflet, style guide, feature considerations, sending pulls, etc.
+
+## Improving Documentation
+
+TODO how to contribute to docs.