Document the type mapping and the recommended pattern for working with
extensions. This documentation was Google-internal (cl/335451491), presumably
because of the reference to static analysis tools that don’t exist outside.
Compared to the internal version, this CL uses “should” instead of “must”.
Change-Id: I9293c10b2f6f91c9546d145c94b52e84f3a7a9c9
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/607995
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Christian Höppner <hoeppi@google.com>
ed7b51feef