RFC: SavedModel Save/Load in 2.x (#34)

* RFC for SavedModel Save/Load in 2.x

* Minor edits and a discussion topic for load() with multiple MetaGraphs

* Tweak to the "Imported representations of signatures" section

* Update "Importing existing SavedModels" with the .signatures change

* Update RFC and add review notes

* Status -> accepted

Author: allenlavoie