Adjusted first coordinates paragraph based on @mservillat's comment
Added mention of IAU precession/nutation models as suggested by Tim Jenness
Added response to @mservillat's question about WCS/coordinates connection, as well as more discussion of future WCS plans
Add mention of and footnote explaining more about the license based on @mservillat's comment
Merge pull request #2 from eteq/astrofrog-second-review-additions

Additions to second review edits