Define .gitignore template

[tobolar]

I haven't found any Modelica template for .gitignore on https://github.com/github/gitignore.
Shall we prepare one? Maybe with a general part and vendor parts?

[dietmarw]

Good idea.

[tobolar]

I've prepared the first draft of gitignore for Modelica, looking several Modelica repos over and reducing (to my knowledge) to the most common setting.
https://github.com/tobolar/gitignore/tree/addModelica, or directly the file Modelica.gitignore

Please feel free to add, delete, modify and whatever to improve this gitignore file. Once finalized, I would trigger the PR.