git reset does not affect commit, but changes index, stage to them

Clear up what happens when you run `git reset` in that it does
not affect the commit, or allow to change it per se, but that it
affects what is sitting in the index and staging as pointed to
by the commit or HEAD pointer mentioned.

Fixes #63
Clarify where git reset moves HEAD, affect on stage, index
Merge pull request #73 from adailey14/63-correct-reset-notes

Rebase of pull request 64 onto latest gh-pages