Clarify FAQ and add 'abbr' tags

[joker314]

Resolves #38

I'm not completely happy with the wording so that's something to review.

• Add <abbr> tags around all the references to "RFC"
• Add a new FAQ section called "Are there any settings I should apply to the file?" (the answer is: serve over HTTPS, set MIME type to text/plain)
• Clarify that you can place the security.txt file at the root of websites and code repositories

Here is what the changes look like:

[nightwatchcyber]

I think that looks fine to me, although we should consider making the language around the root directory and HTTPS stronger.

[joker314]

@nightwatchcyber I've changed "should" to "must" for the HTTPS, especially because the Internet draft has done the same.