CONTRIBUTING.md: initial checkin

[austin987]

This is an initial attempt at adding a contributing document. I would very much appreciate feedback from anyone who contributes and was confused by any part of the process.

CC'ing people who have contributed recently for their feedback: @jre-wine @mihaitodor @mmengden @AndreRH @glintwine @AsciiWolf @hillwoodroc @qwertychouskie @Chiitoo @eli-schwartz @kakurasan

All feedback welcome. I plan to leave this open for at least a couple weeks to allow time for feedback.

[austin987]

You can see a rendered version here: https://github.com/austin987/winetricks/blob/contributing/CONTRIBUTING.md

[kakurasan]

I would like to add the following items ([1], [2], [3] and [4]):

## Making a patch:
* `./tests/shell-checks`: MUST pass (Travis CI verifies)
    * [1] This tool uses checkbashisms (package `devscripts` on Debian-based distributions) and [ShellCheck](https://github.com/koalaman/shellcheck)
    * [2] If ShellCheck fails, see [ShellCheck wiki](https://github.com/koalaman/shellcheck/wiki) and fix/[ignore](https://github.com/koalaman/shellcheck/wiki/Ignore) the error(s)
        * [3] Detailed error information is available in the wiki (e.g. [SC2154](https://github.com/koalaman/shellcheck/wiki/SC2154))

## Sending your patch:
* Commit:
    * Commit should start with ...
        * [4] If you add a new verb, use the format: `verb_name: new verb(, description...)`

[austin987]

I've addressed the feedback given so far by @mihaitodor and @kakurasan

[jre-wine]

Thanks Austin! Just minor nitpicks (for reference, my PR is at https://github.com/austin987/winetricks/pull/1, but discussion should probably happen here):

• in the "Test it" section maybe add some explanation what the tests do (see PR)
• See PR for a minor rewording about the start of commit messages. I think this makes the sentence easier to read.
• What are "Extended git logs"? I guess long commit messages. Do we have to specify how long the first line of the commit message may be, and that the second line should be blank?
• My contributions are not the typical ones, they are often very general and affecting many files. So I often wonder if I should split up my changes (like for patches at winehq.org), or keep everything in one commit. I tend to do the latter here. See PR.

[eli-schwartz]

Looks okay, but consider implementing that instead/also as .github/{ISSUE,PULL_REQUEST}_TEMPLATE.md