add example of defining custom coordinate class
line wrapping and fixed a few minor errors
put full example in its own module
add sgr-example to index
update example to use rotation_matrix
fix sphinx warnings
grammar
change maxdepth to 1
updated whatsnew to mention the sgr example
updated changelog for sgr example and name resolver
Merge pull request #645 from adrn/sgrcoordinates-example (with changelog updates)