Improve documentation

[gsingh93]

I think it would be helpful to document a few things and give some examples, either in the README or in some other location (CHANGELOG.org mentions some of these, but not in enough detail):

• All of the custom properties, i.e. ANKI_NOTE_TYPE
• Creating notes where the subheadings are the field names (from https://github.com/eyeinsky/org-anki/issues/48)
• Adding new note types in org-anki-model-fields

There's probably some other features that aren't documented, but these are the few that I went looking for and didn't find.

[eyeinsky]

Thanks for doing this! While PRs are always welcome I'll try to address these when I get a moment!

[eyeinsky]

I've started improving the readme in update-readme branch.

[gsingh93]

Thanks for working on this :)

After listing all of the features, I think having three separate subsections in the README explaining the three ways of creating cards would be good: the original front/back method, cloze cards, and the recent addition of the field names coming from the subheadings.

After that, I think the new content should essentially be what I put in the original issue, a section for how to create a new note type with org-anki-model-fields (unless #61 gets implemented), a section discussing all of the org-properties that can be added (ANKI_NOTE_ID, ANKI_DECK, ANKI_MATCH, ANKI_NOTE_TYPE, ANKI_TAGS), and then any other defcustom settings that may need a bit more explanation up front.