Fix documentation on --set

[RhetTbull]

Need to improve documentation to make it clear how to use --set

    Thanks for looking. No I did not set that metadata myself, it's what comes out of the box with that particular app. I note that at least for Spotlight searches, it is parsed correctly by mds—even though the format might not be correct or recognized by osxmetadata.

Thanks for the syntax for multiple keywords. I didn't know to use --set keywords foo --append keywords bar. I was doing repeated groups of --set which also somehow does seem to work.

Originally posted by @luckman212 in #83 (comment)