Document .rubyfmtignore and .gitignore behavior

[reese]

Resolves #354

We added support for ignore-files some time ago, and I think some users will find them pretty useful, especially in bigger projects, so adding some docs to the README for them.

[matthewford]

Hi, I'm trying out using .rubyfmtignore with VS Code. However, it looks like the file is not being respected.

[reese]

@matthewford is it respected if you run rubyfmt directly in the root directory?

[matthewford]

@reese I suspect it is.

[reese]

In that case, that sounds like something in the VSCode extension and should probably be filed as an issue there. I don't have a ton of context on how that extension works though.

[reese]

Ah it looks like it passes things through stdin, which is probably why this doesn't work. I had a thought that we should support something like --stdin-filepath that prettier has, which is basically "format these contents but treat them like they're at this file path to handle e.g. file ignores," but we don't have that currently (tracking in #365)