Write "How to contribute"

ContributorGuidelines.md

@@ -17,3 +17,16 @@ Summing up, you have 3 choices where to contribute with MicroPython development:
 * Maintain your own module or port, as part of general MicroPython community  
 
 # How to contribute
+
+Please make sure you read previous sections to understand MicroPython philosophy and approach. We expect contributors will be aligned with them on general matters, because otherwise it would be hard to understand why specific change is proposed at all.
+
+If you're not sure if some change would go along with MicroPython approach, please discuss this change first. We have 2 channels of communication:
+
+* Project tickets
+* [Development forum](http://forum.micropython.org/viewforum.php?f=3)
+
+You may not agree with us on interpretation of specific feature, please share your point of view, and be prepared to elaborate and argue it. But please keep in mind that we necessarily have to be conservative to achieve (and not lose on the way) project aims as stated in "What to contribute" section.
+
+We accept changes using Github pull requests, as the most seamless way for both ourselves and contributors. When you make commits for a change, please follow format of existing commit messages (use "git log" to review them). For considerable changes, we expect detailed, yet formal and concise description in of the change in commit messages. If you make 2 or more considerable changes, they should go in separate commits. The same applies to unrelated changes - for example, don't put formatting changes and actual code changes in one commit. 
+
+When you submit pull request, please make sure that it's understood why you propose this change: what problem it addresses, how it improves MicroPython (and not deteriorates, taking into account stringent constraints of "What to contribute" section). Good commit messages are good way to achieve this, but sometimes it's better to put informal points, colloquial arguments and big example in comments to pull request instead.