add contributing guidelines

CONTRIBUTING.md

@@ -0,0 +1,22 @@
+## Welcome to Processing!
+
+### Bug Report?
+
+We have a page on [troubleshooting](https://github.com/processing/processing/wiki/Troubleshooting) common problems. Check there first!
+
+If you don't find an answer, please let us know by [filing an issue](https://github.com/processing/processing/issues). We can only fix the things we've heard about.
+
+Please keep the tone polite. This project is volunteer work done in our free time. We give it away at no cost. We do this because we think it's important for the community and enjoy it. Complaints that things *suck* are *annoying* or lectures about things that *must* be fixed are... weird things to hear from strangers at best, demotivating at worst. 
+
+### Coding Question?
+
+We host an [online forum](https://forum.processing.org) for coding questions, which is also helpful for general "getting started" queries.
+
+### Want to Help?
+
+Great! The number of contributors on this project is *tiny*, especially relative to the number of users. There are [only 2 or 3 people](https://github.com/processing/processing/graphs/contributors) who actively work on this repository, for instance. We need help!
+
+How to start:
+
+* Issues marked [help](https://github.com/processing/processing/issues?q=is%3Aissue+is%3Aopen+label%3Ahelp) are a good place to start, because they're something that's isolated enough that someone can jump into it without significant reworking of other code.
+* Mind the [Style Guidelines](https://github.com/processing/processing/wiki/Style-Guidelines) when submitting Pull Requests. Otherwise someone else will have to reformat your code so that it fits everything else (or we'll have to reject it if it'll take us too long to clean it up).