Add x-www-form-urlencoded recommendation

[cholmes]

In radiantearth/stac-spec#724 we agreed that it would be good to have x-www-form-urlencoded as a recommendation. But we decided to take it out for RC2 because it is not specified anywhere, and has no explanation as to why someone might use it.

With that text it should easily get back into the spec, but we want to get rc2 / 0.9.0 out asap.

[matthewhanson]

Talked about this in sprint #6, not convinced we really need to do this, but waiting for @philvarner to weigh in.

[m-mohr]

Closing as stale issue for now.