Add Versioning section [ci skip]

README.md

@@ -405,6 +405,22 @@ implementation, you will be personally responsible for providing patches in a
 timely fashion. If critical issues for a particular implementation exist at the
 time of a major release, support for that Ruby version may be dropped.
 
+## Versioning
+
+This library aims to adhere to [Semantic Versioning 2.0.0][semver]. Violations
+of this scheme should be reported as bugs. Specifically, if a minor or patch
+version is released that breaks backward compatibility, that version should be
+immediately yanked and/or a new version should be immediately released that
+restores compatibility. Breaking changes to the public API will only be
+introduced with new major versions. As a result of this policy, you can (and
+should) specify a dependency on this gem using the [Pessimistic Version
+Constraint][pvc] with two digits of precision. For example:
+
+    spec.add_dependency 'twitter', '~> 4.0'
+
+[semver]: http://semver.org/
+[pvc]: http://docs.rubygems.org/read/chapter/16#page74
+
 ## Additional Notes
 This will be the last major version of this library to support Ruby 1.8.
 Requiring Ruby 1.9 will allow us to [remove][class_variable_get]