Our non-technical documents are currently floating around loosely in our
project's root, making it harden than necessary to discover what one is
searching for. We do have a "docs/" directory, though, which serves
exactly that purpose of hosting documentation.

Move our non-technical documentation into the "docs/" directory. Adjust
all links to these documents.

While we try to conform to a certain commit message style, this style
has never been documented anywhere. Document it such that new
contributors do not have to go through another needless iteration of
their pull requests just to fix up commit messages.

Also various formatting, wrapping and capitalization tweaks.

This updates CONTRIBUTING to reflect the changes to use the
master branch and make explicit recommendations about updating
CHANGELOG.md and providing test coverage.  Also, this includes
some old updates to PROJECTS.md that I wrote to expand the list
of projects, pulling in things from old feature requests.

* Make GIT_INLINE an internal definition so it cannot be used in
  public headers
* Fix language in CONTRIBUTING
* Make index caps API use signed instead of unsigned values

and a couple of other minor doc fixups.