Document WCS / Header "issue" and "best practices"

[cdeil]

@keflavich I think most astronomers don't know about the Header <-> WCS conversion issues and it would be useful to have a short docs page briefly describing the issue and giving a code snippet or two what the "best practice" is to convert back and forth (is it the one listed here?)).

Do you think it would make sense to add this in an extra RST file?
If you like it could also be the "API guideline" for imageutils functions and be more developer oriented.