Add recommended citation to README & website

[adamltyson]

Although following #728 there's a citation CFF file, many people are not familiar with these (or won't see it). We should make sure that how to cite datashuttle is clear and easy.

[JoeZiminski]

Maybe on the github README and Install section on the website)? Or possibly the 'Get started' or 'Community' pages

[adamltyson]

I'd say on the homepage because:

• The paper is also useful reading when you're starting out
• At the point that you're writing a paper, chances are you aren't looking on the installation or getting started pages