[docs] add configuration usage

[marcfranquesa]

First of all, thank you! I found this formatter recently and I am really liking it.

I think it would be nice to add documentation on the available flags to change the settings (i.e. --tab 4). Also relating to this issue, when the config file is implemented to document the usage. Could be a section added to the README? I can start up an initial draft.

Also opened a PR to include tex-fmt in the Mason registry, hope that is ok!

[WGUNDERWOOD]

Now that tex-fmt is becoming a little more complex, I think this is a good idea! Perhaps we can keep the "Usage" section in the README as it is now, containing just the most useful flags. We can then add a new section called "Options" at the very bottom of the README, documenting each flag in more detail. Feel free to make a draft PR and I can take a look.