Add a link to the OAUTHBEARER spec on the SASL mechs page.

Author: sadiepowell

docs/sasl-mechs.md

@@ -8,6 +8,7 @@ page-header: >
 IRC SASL authentication primarily uses the same mechanisms as SASL in other protocols. Most commonly:
 
 * [PLAIN](https://tools.ietf.org/html/rfc4616) as defined by RFC 4616
+* [OAUTHBEARER](https://tools.ietf.org/html/rfc7628) as defined by RFC 6728
 * [EXTERNAL](https://tools.ietf.org/html/rfc4422#appendix-A) as defined by RFC 4422
 * [SCRAM-SHA-256](https://tools.ietf.org/html/rfc7677) as defined by RFC 7677