Add text pointing to full change list.

Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3606)

CHANGES

@@ -2,6 +2,11 @@
  OpenSSL CHANGES
  _______________
 
+ This is a high-level summary of the most important changes.
+ For a full list of changes, see the git commit log; for example,
+ https://github.com/openssl/openssl/commits/ and pick the appropriate
+ release branch.
+
  Changes between 1.1.0f and 1.1.1 [xx XXX xxxx]
 
   *) In the UI interface, make it possible to duplicate the user data.  This