Add/reorganize contributor documentation (#4768) ...

* Move contributing/CoC to GitHub folder

Cleans up the root directory. Alternatively, can use a `docs/` folder,
but I figured that might get confused with API docs.

Ref: https://blog.github.com/2016-02-17-issue-and-pull-request-templates/
Ref: https://help.github.com/articles/adding-a-code-of-conduct-to-your-project/

* Add support links to CONTRIBUTING.md

I only went back to August 2017, but here were a few issues that related
more to usage (or could have been solved by looking up docs).

Fixes #4699
Fixes #4671
Fixes #4614
Fixes #4416

* Add GitHub issue template

* Add empty GitHub pull request template

* Update README.md

- Removes broken link (Fight for Everyone)
- Updates CDN link to v4.7.1
- Uses .github/CONTRIBUTING.md reference