Updated docs for 1.0, added developer docs

the docs-support/ and docs/ directories contain all the files
and scripts needed to produce a nice HTML of the developer docs.

Developer docs include specifications for mdmerge, Python
virtual environment setup and usage, and Git procedures
(including release procedures) for the project.

See the tools*.mmd files to know what tools are required to
build the developer docs.

Cleanup CHANGELOG and README for 1.0 release.
Cleaned up grammar, spelling. Removed the --pre flag from install
instructions. Added info about developer docs.