Better document "Using with Postman" to `README.md`

[mobileskyfi]

It be better if all the docs are in one places. The Mikrotik forum has some instructions for using the RAML schema with Postman:
https://forum.mikrotik.com/viewtopic.php?t=199476

But it better if it was just shown in the markdown here.

[mobileskyfi]

Awaiting fix for Postman here postmanlabs/raml1-to-postman#84