README: add missing link to OpenSSL 3.0 manual pages

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17855)

README.md

@@ -126,6 +126,7 @@ The manual pages for the master branch and all current stable releases are
 available online.
 
 - [OpenSSL master](https://www.openssl.org/docs/manmaster)
+- [OpenSSL 3.0](https://www.openssl.org/docs/man3.0)
 - [OpenSSL 1.1.1](https://www.openssl.org/docs/man1.1.1)
 
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