Update contributing readme

Add section about github label system

CONTRIBUTING.md

@@ -18,6 +18,23 @@ If you want to contribute code:
 
 Please note the special instructions for contributing new source code files, asset files, or user-facing strings to the [iOS SDK](platform/ios/DEVELOPING.md#contributing) or [macOS SDK](platform/macos/DEVELOPING.md#contributing).
 
+### Github Issue Labels
+
+Our labeling system is
+
+ - **minimalistic:** Labels' usefulness are inversely proportional to how many we have.
+ - **objective:** Labels should be objective enough that any two people would agree on a labeling decision.
+ - **useful:** Labels should track state or capture semantic meaning that would otherwise be hard to search.
+
+We have divided our labels into categories to make them easier to use.
+
+ - platform (purple)
+ - type (blue)
+ - actionable status (red)
+ - non-actionable status (grey)
+ - importance / urgency (green)
+ - topic / project / misc (yellow)
+
 # Code of conduct
 Everyone is invited to participate in Mapbox’s open source projects and public discussions: we want to create a welcoming and friendly environment. Harassment of participants or other unethical and unprofessional behavior will not be tolerated in our spaces. The [Contributor Covenant](http://contributor-covenant.org) applies to all projects under the Mapbox organization and we ask that you please read [the full text](http://contributor-covenant.org/version/1/2/0/).