New rule for Recommended Extensions: no @Unstable API usage

Hi devs,

Simon just made me realize that it would be a bad idea to recommend an extension that uses @Unstable APIs because if these APIs are modified, then the extension is broken and even if a new version is released then our users will be broken until they upgrade.

Thus I’d like to propose to add this rule (which is kind of obvious but I think it’s better to write it down) to the list of rules at