Add .gitignore

i've created a .gitignore file for the MacOS .DS_Store file. if i use this addon as git submodule, the subproject is "dirty" and you need to delete the .DS_Store before you can push...

```
-Subproject commit 580a7a678bde19f2780c4bd38ea43032e5677295
+Subproject commit 580a7a678bde19f2780c4bd38ea43032e5677295-dirty
```