Minor howto and release doc additions

howto/code/voltha-protos.rst
----------------------------
  - Document virtualenv setup and lint command.
  - Include notes about header longintrepr.h, early VGC related failure.

howto/release/overview.rst
--------------------------
  - Review release overview docs for content.
  - URL will be linked to from the wiki release tracking page.

Change-Id: I1f45834bb445ba2cc31de33cc4b45653db91f111
3 files changed