document that RFC 8879 is supported

Author: tomato42

README

@@ -39,6 +39,7 @@ Functionality implemented include:
  - FFDHE prime/group negotiation (RFC 7919)
  - Heartbeat Extension (RFC 6520)
  - Record Size Limit (RFC 8449)
+ - TLS Certificate Compression (RFC 8879)
 
 
 tlslite-ng aims to be a drop-in replacement for tlslite while providing more