add beginnings of merchant API docs

curl "https://sign2pay.com/api/v2/merchants.json" \
  -H "Content-Type: application/json" \
  -d "{\"merchant\":{\"email\":\"widgets-inc@example.com\",\"name\":\"Widgets, Inc\"}}"

I am bringing this under the v2 namespace to make a clean break with
v1. The idea is to move everything from v1 to v2 and document it
a little better.