Add documentation about using local files in custom schemas

[Zachiah]

Closes: #7

If there are any other details I should mention or edits I should make please let me know. Thanks

[Zachiah]

Hi @b0o just checking in, is there anything stopping this from getting merged?

[Zachiah]

I force pushed because I didn't want to have a bunch of commits in the history for this. Hopefully that's fine

[b0o]

Thank you!