Note about api deprecation

Author: ryanflorence

CONTRIBUTING.md

@@ -14,6 +14,15 @@ that also:
 - Updates examples
 - Updates the docs
 
+## API Deprecation
+
+Introduce new API in minor versions, remove old APIs in major versions.
+
+For example, if the current version is 2.3 and we change `<Router/>`
+`<Rooter/>` the next release will be 2.4 that contains both APIs but
+`<Router/>` would log a warning to the console.  When 3.0 is released,
+`<Router/>` would be completely removed.
+
 ### Development
 
 - `npm test` will fire up a karma test runner and watch for changes