sso: add attribution to our readme

Author: danbf

README.md

@@ -1,5 +1,7 @@
 # sso
 
+[sso](https://github.com/buzzfeed/sso) is built on top of Bitly’s open source [oauth2_proxy](https://github.com/bitly/oauth2_proxy) 
+
 > See our launch [blog post](https://tech.buzzfeed.com/unleashing-the-a6a1a5da39d6) for more information!
 
 [![CircleCI](https://circleci.com/gh/buzzfeed/sso.svg?style=svg)](https://circleci.com/gh/buzzfeed/sso)
@@ -69,5 +71,9 @@ If you come across any security vulnerabilities with the **sso** repo or softwar
 
 **sso** is actively maintained by the BuzzFeed Infrastructure teams.
 
+## Notable forks
+
+ - [pomerium](https://github.com/pomerium/pomerium) an identity-access proxy, inspired by BeyondCorp.
+
 [docker_hub]: https://hub.docker.com/r/buzzfeed/sso/
 [sso_survey]: https://docs.google.com/forms/d/e/1FAIpQLSeRjf66ZSpMkSASMbYebx6QvECYRj9nUevOhUF2huw53sE6_g/viewform