Add notes about using CSS "templates".

[gaand]

This recurs frequently enough that we should address it at least to say "don't".

[gaand]

Is this addressed by #66 ❓

[jrhorn424]

I'm not sure. If you say to me: "don't use a css template", I hear "don't download this bootstrap theme and throw it into your project and expect it to work".

That's not the same thing as #66, which to me says: "hey, as long as you're reading this, don't muck with it, ok?".

Not adding a <link> isn't the same thing as replacing the file wholesale.