rfc: Indicate APPENDLIMIT support

Author: cmouse

docs/core/summaries/rfc.md

@@ -99,6 +99,7 @@ This is a non-exhaustive list of RFCs that Dovecot implements/supports.
   Quick Mailbox Resynchronization (QRESYNC) (updated RFC 4551 and RFC 5162)
   ([[rfc,7162]])
 - IMAP4 Non-synchronizing Literals (updated RFC 2088) ([[rfc,7888]])
+- IMAP4 APPENDLIMIT Extension ([[rfc,7889]])
 - IMAP Extension for STATUS=SIZE ([[rfc,8438]])
 - IMAP4 Extension for Returning MYRIGHTS Information in Extended LIST ([[rfc,8440]])
 - IMAP “$Important” Keyword and “\\Important” Special-Use Attribute