Some wordy changes and other ideas

Looks good! I made some minor language changes, all of which you should feel free to change back, and some lists in the text ('this, that, and the other') used a serial comma, while others didn't. I think got them all (added the serial comma). Nothing Earth shattering. I've added some other thoughts back in a comment on #103.

One thing to call out: presumably we can endorse our own apis, and I changed first sentence of last para of section 1 to start: 

 > The inclusion of services in this document that are outside of the IIIF domain _MUST NOT_ be interpreted as endorsement ...