Document that `-I` picks up on `dirname(file)` automatically

[sirosen]

I was trying to figure out how much I needed to script/wrap to have cog to grab a ./_cog_helpers.py file from the directory where a file lives with -I. It then struck me that maybe . should often be added to the path, so I figured either it was a feature already or I should suggest a flag for it.

Looks like it already is the behavior! https://github.com/nedbat/cog/blob/86d8f30cf8ea4833b3a738bb0baa1238f850d931/cogapp/cogapp.py#L674-L676

I found my way, but I think the docs should call this out explicitly. I'm happy to post a PR. What do you think of adding a one-sentence note to https://cog.readthedocs.io/en/latest/running.html#miscellaneous ?

[nedbat]

Sure, sounds great!