Add info to upgrade manual

[grevus]

Q	A
Documentation	yes
Library version	9+

Please, add information that you changed library for Base32 encoding to this file:

otphp/UPGRADE_v8-v9.md

-        $encoded_secret =  Base32\Base32::encode($secret);
+        $encoded_secret =  ParagonIE\ConstantTime\Base32::encodeUpper($secret);

[Spomky]

Good catch.

As the version 8.3 is concerned by this change, I modified the README file instead of the otphp/UPGRADE_v8-v9.md one.

Thanks

[grevus]

Thanks