Add index page for HTTP API

setup/doc/api.md

+osTicket API
+============
+
+The osTicket API is implemented as (somewhat) simple XML or JSON over HTTP.
+For now, only ticket creation is supported, but eventually, all resources
+inside osTicket will be accessible and modifiable via the API.
+
+Authentication
+--------------
+
+Authentication via the API is done via API keys configured inside the
+osTicket admin panel. API keys are created and tied to a source IP address,
+which will be checked against the source IP of requests to the HTTP API.
+
+Wrappers
+--------
+
+Currently, there are no wrappers for the API. If you've written one and
+would like it on the list, submit a pull request to add your wrapper.
+
+Resources
+---------
+
+- [Tickets](api/tickets.md)