Add contributor guidelines

[joaander]

This PR adds contributor guidelines, issue, and pull request templates to facilitate external contributions to hoomd-examples. The file OUTLINE.md is currently a placeholder to be later filled out with the contents of #2.

As hoomd-examples will be used closely in conjunction with HOOMD, I am including it under the same license and contributor agreement as the parent project going forward.

glotzerlab reviewers, please go ahead and make any obvious fixes or improvements directly and push them to the branch.

[bdice]

Awesome, I really like this layout for the issue / PR templates.

A few items for consideration:

• I'd like to see the README.md contain the list of examples / table of contents, rather than OUTLINE.md. In my mind, the README.md should contain a couple introductory remarks (similar to the existing README.md), followed by a full Table of Contents for all examples. I think a long README.md is appropriate for this repository - it simplifies the repository organization, makes everything "one click away," and ensures that all content is highly visible to people browsing the repo's main page.

• I'd rather use a naming pattern like examples/Initialization/Create-Lattice.ipynb or perhaps examples/01-Initialization/02-Create-Lattice.ipynb. Any kind of subdirectory structure would be good, to avoid a massive repository root directory. I would aim for < 15 files in the repository root like https://github.com/glotzerlab/freud-examples.

• I would avoid relying on lexicographical ordering for notebooks. It would be annoying if we decided a new example logically fits somewhere between 1201 and 1202 and had to rename examples 1202 through 1234 ➡️ 1203 through 1235. The tutorials probably won't really be that linear, anyway. I would prefer the ordering to be encoded by the order of links in the README.md rather than hardcoded filenames. You can see an example of this in freud-examples, except pretend we used README.md instead of index.ipynb: https://github.com/glotzerlab/freud-examples/blob/master/index.ipynb

• We may want an index.ipynb that duplicates the Table of Contents of all examples, since Markdown files don't play nice with Jupyter Notebook (but they are rendered properly in Jupyter Lab). I wish I knew a better way than to duplicate that content...

[joaander]

New examples may add digits to insert between existing examples and avoid O(N) renaming:

▶ ls
00-ex1  011-ex-new  01-ex2  02-ex3  03-ex4