Add a link to wiki 3.0 upgrade notes

Provide a link to the 3.0 upgrade notes so that users of the alpha/beta/
final releases can understand the differences between this version and
previous versions.
Reviewed-by: NNicola Tuveri <nic.tuv@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/11587)

README.md

@@ -118,6 +118,9 @@ document.
  * [NOTES.VMS](NOTES.VMS)
  * [NOTES.WIN](NOTES.WIN)
 
+Specific notes on upgrading to OpenSSL 3.0 from previous versions, as well as
+known issues are available on the OpenSSL
+[wiki](https://wiki.openssl.org/index.php/OpenSSL_3.0).
 
 Documentation
 =============