Add to docs how to get Well Known Types

[feinstein]

As can be seen here and in many many other issues and stack overflow questions, the protobuf Well Known Types are not automatically generated by the Dart protobuf plugin. I saw some claims that Dart is the only plugin that behaves this way. I understand this is a technical decision from the team, but at least the docs should include how to get the WKT generated and working.

I have hit many caveats, until I finally understood what was going on. If it helps, my description is here, and I believe the docs should include something similar.