Include Web API Guidelines Annex in template

[fterpstra]

The OGC Web API guidelines tasks force will add, in this repository, the template for a Web API Guidelines Annex for OGC API standards after the March 22nd closed OAB meeting.

[cmheazel]

The annex has been posted to the API Guidelines repository. Scott Simmons took the action to add it to the template for standards. (OAB 3/22/21)