docs: improve documentation around adding media types

Author: dougwilson

README.md

@@ -76,13 +76,19 @@ and the values being an object with the following keys:
 
 To update the build, run `npm run build`.
 
-## Adding Custom Media Types
+### Adding Custom Media Types
 
 The best way to get new media types included in this library is to register
 them with the IANA. The community registration procedure is outlined in
 [RFC 6838 section 5](http://tools.ietf.org/html/rfc6838#section-5). Types
 registered with the IANA are automatically pulled into this library.
 
+If that is not possible / feasible, they can be added directly here as a
+"custom" type. To do this, it is required to have a primary source that
+definitively lists the media type. If an extension is going to be listed as
+associateed with this media type, the source must definitively link the
+media type and extension as well.
+
 [coveralls-image]: https://badgen.net/coveralls/c/github/jshttp/mime-db/master
 [coveralls-url]: https://coveralls.io/r/jshttp/mime-db?branch=master
 [node-image]: https://badgen.net/npm/node/mime-db