godoc: add README

Explain how to make changes to the project's static files and get them compiled
in to the godoc binary.

Change-Id: I12e0c50bf31925b3f211b6d0dd3e5f8cff547f32
Reviewed-on: https://go-review.googlesource.com/34925
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed