.gitigore: ignore AppCode's .idea folder

[chzhuo]

Checklist

• tests and code linting passes
• a test and/or benchmark is included
• documentation is changed or added
• the commit message follows commit guidelines

Affected core subsystem(s)

Description of change

[Fishrock123]

We may accept this due to past history, but you should still add this to you global gitignore so that it doesn't conflict with other projects. Instructions at https://help.github.com/articles/ignoring-files/#create-a-global-gitignore

[jasnell]

+0 on this. No objections in general but also not convinced it's necessary

[sam-github]

I'm -1, I think we should encourage contributors to set up git correctly on their system, and that setting git up to ignore your personal editor's files is correct.

[gibfahn]

-1 for the same reasons, there are a bunch of different .directories we'd need to ignore to cover everything (one for every IDE that does this at least), and all the other things like .tags for ctags. If you're using these IDEs you almost certainly want them ignored in your other git projects as well, so a global .gitignore is probably a better way to go.

That said, maybe what would be more helpful is some documentation explaining that you can add a global .gitignore, maybe something in CONTRIBUTING.md?

[addaleax]

I think this can be closed now that 15cc7c0 converted our .gitignore to a whitelist for dotfiles instead of a blacklist. Thanks for the PR, though!