Create extension_conventions

[fcuantico]

This is the conventions followed in NWChemEx for different file types

Is this pull request associated with an issue(s)?
No, this associated with adding content to the documetation
Description
Add the description what files extensions should be followed

TODOs
For draft pull requests please include a list of what needs to be done and check
off items as you complete them.

[ryanmrichard]

I think this is supposed to be part of the other PR. The edit online thing only works if you're changing a single file. If you're going to be touching multiple files you have to go through the normal PR process (clone the repo, make a branch, add your changes, push the branch, make a PR off the branch).

[fcuantico]

Oh ok let me see how that works thanks Ryan!