Merge pull request #3 from nslater/mode-line-change

Change "wm" to "Margin"
Merge pull request #4 from nslater/readme-formatting

Readme formatting
Merge pull request #5 from nslater/fix-hook-doc

Change text-mode to text-mode-hook
README.md: minor tweaks

Improve language and formatting.

Correct the copyright notice: the file is *not* part of GNU
Emacs. (That wording is used specifically when the file is actually
distributed with Emacs.)