That entire repository is documentation for the API. I don't know why it isn't in the faq, or a top link in the help section.
I'm trying to create a custom remote based on the built-in Navigation remote using the online tutorial (www.unifiedremote.com/tutorials/how-to-create-a-custom-keyboard-shortcuts-remote).
Unfortunately, nothing looks quite matches the tutorial, and the end result doesn't show in the list of available remotes (either locally via the server manager, or on my android device). I did copy the keyboard.lua file into the custom remote's folder (since it's mentioned in the remote.lua file).
My only modification is to replace the space key with shift.