Contributing: More/better contribution information for first-timers

[ao5357]

Per @astearns 's thread on www-style of February 14, 2018, "[CSSWG] New issue labels in GitHub", specifically @frivoal 's contention that there be more documentation regarding resolutions, editor reviews, and flagging for the agenda when it comes to Issues/PRs.

In order to streamline the process, I propose the following:

1. Create a PULL_REQUEST_TEMPLATE file that contains the pertinent attribution information from CONTRIBUTING.md, as well as steps for submitting
2. Tweak CONTRIBUTING.md to be entirely about contributing, and to have a strong document outline
3. Include a copy of the W3C's CEPC in the repository as CODE_OF_CONDUCT.md in the repo root (per GitHub de facto standards)

I was curious what you thought about setting ISSUE_TEMPLATE as ISSUE_TEMPLATE.md (and similarly for the PR template). Additionally to do similar and rename README.markdown to README.md? I know this is all small stuff that's not related to CSS specs in any way, but I figured I'd ask while I'm in there trying to pitch in.

Initial PR to follow shortly. Thank you for your time and consideration.

(cc. @AmeliaBR as she was also party to the thread)

[ao5357]

Not sure where it might fit:

• The readme
• CONTRIBUTING.md
• An additional doc/wiki page

...but there is some information for a first-time contributor that could make understanding the process a bit smoother:

• What bikeshed is, and where you can find it
• How to find/search/clone the major browser CSS engines
• What various acronyms and initialisms mean
• How web-platform-tests applies to stylesheet specs

Are these topics worth including in this issue?

[fantasai]

I think these are all good topics to cover, but probably a better location would be the (currently mostly-useless) CSSWG website. https://wiki.csswg.org/tools has a bunch of the tooling-related info,though.

[elmvguia]

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