[Docs] Update release notes for 5.5.1.

docs/ReleaseNotes.txt

@@ -15,13 +15,16 @@ Version 5.5.1
 **[Server]** Avoid SEGV when client tries to access file after deferred closed.
 **[XrdHttp]** The server certificate is renewed by the Refresh thread of the
               XrdTlsContext object.
+**[XrdHttp]** Fix a segv happening when a client sends a first line starting
+              with a space.
 **[XrdTls]** Shutdown the socket if a SSL error happens when trying to accept
              a connection.
 
 + **Minor bug fixes**
 **[Apps]** Avoid SEGV when asking for help.
 **[XrdCl]** copy job: fix memory leak (buffers not queued on error).
 **[Server]** Add O_RDWR open flag when creating file to avoid fs issue.
+**[Server]** Properly handle opaque info for fsctl operations.
 **[XrdHttp]** Allow VO names with spaces and other quoted chars.
 **[XrdCl]** LocalFileHandler: fail gracefuly on overloaded machines.