add link to api docs for conditional requests

github/doc.go

@@ -77,6 +77,9 @@ https://github.com/gregjones/httpcache, which can be used in conjuction with
 https://github.com/sourcegraph/apiproxy to provide additional flexibility and
 control of caching rules.
 
+Learn more about GitHub rate limiting at
+https://developer.github.com/v3/#conditional-requests.
+
 Creating and Updating Resources
 
 All structs for GitHub resources use pointer values for all non-repeated fields.