Emacs: Use completing-read When Prompting

[sshaw]

Hi,

Just installed your package and noticed that you don't use completing-read. Any reason why?

I can submit a pull request with the changes, but the source indicates that it was auto generated and I don't see where this is done.

Possibly related to #1

[andreineculau]

@sshaw No good reason, just procrastination - and thus the existence of #1.
Checkout the dev branch, update generators/emacs/*.tpl and send a pull request

Your contribution is much appreciated!
I'll also add Added a CONTRIBUTING.md to clarify some things - thanks for reminding me!