Adopt a Code of Conduct

[vhf]

I think it's a shame that the second most popular github repository does not have a code of conduct and I intend to fix this. We never needed a code of conduct, we don't want to need a code of conduct and that's why we need a code of conduct.

I submit this as a PR instead of committing directly to master because I want to allow some debate on this issue. I will merge this PR in a week or so, if there is no good case against it.

[onebree]

Although this repo is just a set of links, I think a COC is a great idea. Like you said, this is a popular repo, that is often hit by spammers. While a COC will not prevent spammers, at least rules are still implemented.

1. Why did you choose to adopt this particular COC?
2. The rendered Markdown treats each LOC as a new line of text. I think the 2 spaces following each line causes this. If the COC becomes a markdown file, I think the text should flow as expected on a webpage.

[vhf]

Thanks for your input.

1. I had a look at the most popular COCs and chose this one. This resource helped me. This PR is also here to discuss if the proposed COC is the one we should adopt. I don't have strong feelings towards this particular COC and am open to dialogue.
2. Fixed - thanks!

[onebree]

1. I do not have a strong opinion towards any one COC, either. Honestly, I did not take the time to read every line of this one, because, like mentioned before, we are just a repo of links. COC really comes into play during conversations, which are actually handled well already.
2. You did not need to write 1 line per paragraph. Markdown renders fine if a line (but mid-paragraph) ends with 0-1 spaces. Otherwise, it is treated as a line break. But hey, whatever :-) Still looks good.

[eshellman]

Thanks for adding this.