Add draw.io XML. Update README.md

Add dnote-explanation.ml for loading into https://www.draw.io for generating
explanation images on how the d-note application works. License the XML itself
under the AGPL, and license the generated images under the CC-BY-NC-SA.
explain how the xml file works with draw.io
clarification
some image updates. show final ciphertext is saved as base64
Remove base64 from the saved notes

Base64 isn't really getting us anything, and it's only adding unnecessary code
and using up more disk space (almost 40% more) than just writing the binary
directly. Removing this will break previously running installs with notes that
have not yet been decrypted.
some image updates. show final ciphertext is saved as base64
Merge branch 'master' of github.com:atoponce/d-note
remove base64 on final ciphertext image
remove duplicate logic. clean up show_post()
Rename of some variables. Always use salts.

Rather than generate a 112-byte string from PBKDF2, the AES and HMAC keys are
generated from the nonce that creates the URL and the AES and HMAC salts
provided in dconfig.py. As a result of this change, some variables have been
renamed for better self-documentation.

Also, to help with additional code self-documentation the 'key' variable has
been renamed to 'aes_key'.
fix my ocd
order variables satisfying my ocd
one more variable change to satisfy my never-ending ocd
fix ordering bug as my previous variable ordering ocd got a bit aggressive
update docstrings and other cleanup
remove silly uncessary logic
some docstring cleanup and consistencies
create_url() docstring accuracy update
decode_url(url) docstring accuracy update
Reworked python docs to better represent the actual code