Add infra/bots/Makefile to simplify test/train

eg.
$ make test
or
$ make train

Update docs accordingly.

Bug: skia:
Change-Id: I069fa686833f0a78f398bd85eb8bb44025cae0d5
Reviewed-on: https://skia-review.googlesource.com/13327
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
8 files changed