README: Add "GopherJS Development" section. (#520)
README: Add "GopherJS Development" section. (#520)

The linked document covers things like code style, code generation,
and other notes and expectations for this project.

Resolves #517.
albrow/gopherjs,gopherjs/gopherjs,albrow/gopherjs,albrow/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs