Update README to reflect URI changes

[aaronbieber]

The README needs to talk about the two versions of org-protocol URI and explain how to configure the emacsclient path if it differs from /usr/local/bin/emacsclient.

For the weary traveler: create an ini-format file called ~/.orgprotocol.ini and in it, place:

[emacsclient]
path=/the/path/to/your/emacsclient

The path should include the executable name, i.e. emacsclient.