Simplified the templates directory, added writing prompts

It was confusing to have so many different files in the template
directory, both to authors and readers of the proposals. This
commit simplifies the directory of templates down to one 'index.md'
and fills it out with sections that contain writing prompts taken
from JEP-2 combined with Henrik's previous example text.
Merge pull request #20 from ahpook/master

Simplify the templates directory; add writing prompts