update contributing guide

CONTRIBUTING.md

@@ -8,11 +8,20 @@ Contributing to Leaflet
 
 ## 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.
+Third-party patches are absolutely essential on our quest to create the best mapping library that will ever exist. However, they're 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.
+Before reporting a bug on project's [issues page](https://github.com/CloudMade/Leaflet/issues), first make sure that your issue is caused by Leaflet, not your application code (so you always pass correct arguments to methods, etc.). Second, search the already reported issues for similar cases, and if it's already reported, just add any additional details in the comments.
+
+After you made sure that you've found a new Leaflet bug, here are tips for creating a helpful report that will make fixing it much easier and quicker:
+
+ * Write a **descriptive, specific title**. Bad: "Problem with polylines". Good: "Doing X in IE9 causes Z".
+ * Include **browser, OS and Leaflet version** info in the description.
+ * Create a simple **test case** that demonstrates the bug (e.g. using [JSFiddle](http://jsfiddle.net/)).
+ * Check whether the bug can be reproduced in other browsers.
+ * Check if the bug occurs in the stable version, master, or both.
+ * Bonus: if the bug only appears in the master version but the stable version is fine, use `git bisect` to find the exact commit that introduced the bug.
 
 ## Contributing Code