[web-annotation] Protocol should include Accept-Post requirement from LDP

azaroth42 has just created a new issue for 
https://github.com/w3c/web-annotation:

== Protocol should include Accept-Post requirement from LDP ==
The relevant part of the LDP spec:
> 5.2.3.13 LDP servers that support POST must include an Accept-Post 
response header on HTTP OPTIONS responses, listing POST request media 
type(s) supported by the server.

e.g.
```
Accept-Post: application/ld+json, text/turtle
```
Means I accept POSTing JSON-LD and Turtle to create new annotations.

See https://github.com/w3c/web-annotation/issues/85