README.md: move "Getting Started" before "Getting Help"
README.md: add "Table of Contents"
README.md: assimilate INSTALL.md

A Table of Contents is better than two files.
README.md: minor improvements
magit.texi: remove FAQ and ChangeLog

Remove the sparse FAQ and the outdated one-entry ChangeLog;
and instead link to the more up-to-date online information.
magit.texi: update information about minimal Emacs and Git
magit.texi: remove outdated "Developing Extentsions" section

That will come back, but not until after the api has stabilized.
magit.texi: recommend against the "rewrite" functionality
magit.texi: improve "Introduction" a bit
magit.texi: thank every contributor; maintainers by name

Thank you guys and gals!
synchronize magit.texi and README.md
Makefile: assimilate AUTHORS.md.in
AUTHORS.md: update list of contributors
AUTHORS.md: also link to contributor list on github
Merge branch 'pr/redoc'