deprecate pre-2.0 add/subtract
Simplify readme

 * Move the changelog to its own file
 * Make badges look like moment-timezone
Move readme simplification

 * Rename readme.md to README.md because apparently that's the cool
 thing to do
 * Move more stuff into contributing.md.
 * Make CONTRIBUTING.md a little less yelly because the default branch
 thing seems to be working.
more simplification
fixed spelling
Merge pull request #1765 from icambron/simplify_readme

Simplify readme
deprecate pre-2.0 add/subtract
fix todo
cleanup
Merge pull request #1763 from icambron/add_subtract_order

deprecate pre-2.0 add/subtract