Add a README.md because of Sourcehut

Sourcehut doesn't support org-mode README files, so I made a markdown version.
Add instructions for installing with MELPA
Making shell-command-to-string call more robust

Using `shell-quote-argument' on the arguments passed into the ddate
command, and also using `user-error' instead of `error' on bad user
Removed rx, use regex strings instead

I got caught in macro expansion timing heck. This is how I escaped.
Add unit tests and make ddate more robust

Adding unit tests forced me to look at some error correction and edge
cases. Improved reliability of ddate.el significantly. There was much
Updates to satisfy package checking scripts

I used package-lint, checkdoc, etc. Then I addressed the recommendations.
Add a default format for ddate

This format tries to keep it all on one line when there's a
holiday. Nicer for the dashboard.
Add an example with emacs-dashboard

Show how to use ddate with the Emacs dashboard. Clever!
Added a README file

Okay okay this needs more work.