KNOWN_BUGS: GnuTLS backend skips really long certificate fields

Closes #762

docs/KNOWN_BUGS

@@ -27,6 +27,7 @@ problems may have been fixed or changed somewhat since this was written!
  2.1 Hangs with PolarSSL
  2.2 CURLINFO_SSL_VERIFYRESULT has limited support
  2.3 DER in keychain
+ 2.4 GnuTLS backend skips really long certificate fields
 
  3. Email protocols
  3.1 IMAP SEARCH ALL truncated response
@@ -213,6 +214,13 @@ problems may have been fixed or changed somewhat since this was written!
  Curl doesn't recognize certificates in DER format in keychain, but it works
  with PEM.  https://curl.haxx.se/bug/view.cgi?id=1065
 
+2.4 GnuTLS backend skips really long certificate fields
+
+ libcurl calls gnutls_x509_crt_get_dn() with a fixed buffer size and if the
+ field is too long in the cert, it'll just return an error and the field will
+ be displayed blank.
+
+
 3. Email protocols
 
 3.1 IMAP SEARCH ALL truncated response