Update README.md

Minor typos && HTML formatting for variables and keys (this makes the
source uglier since GHM doesn't support definition lists, but the output
is a little more readable IMHO. This fixes #22.
Added ability to permanently delete posts

This adds a command "D" (capital D) that can be
used to delete a post from the repository entirely.
Useful in cases where you want to abandon work and
write a new post instead. Fixes #23.