docs: add ENG-OVERVIEW.md document (#182)
docs: add ENG-OVERVIEW.md document (#182)

* Added ENG-OVERVIEW.md document.
This is a meaty skeleton to be fulled out more.

* docs: improve architecture overview

* Further updates to the ENG-OVERVIEW.md
  - tightend wording.
  - Add Browser and Server sections.

* A few more changes to ENG-OVERVIEW.md.  Sigh.

* docs: minor formatting of eng-overview

google/fusion-tables-drive-export,google/fusion-tables-drive-export