godoc: add Corpus.ReadIndexFrom, which will be useful for creating a split indexing/frontend godoc instance.
godoc: add Corpus.ReadIndexFrom, which will be useful for creating a split
indexing/frontend godoc instance.

R=sameer, sameer
CC=golang-codereviews
https://codereview.appspot.com/78850043

karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools