add ".idea" dir to .gitignore

[grummbeer]

No description provided.

[amenk]

Thank you for the pull request. I rather would prefer everybody uses their global git ignore - do you know how this works? otherwise we need to put an ignore for each IDE used.

[amenk]

for future reference: https://docs.github.com/en/get-started/getting-started-with-git/ignoring-files#configuring-ignored-files-for-all-repositories-on-your-computer

[grummbeer]

Danke für den Link.