Merge pull request #730 from Unity-Technologies/develop-ignore-plugins

Add ignore for plugins folder

.gitignore

 /unity-environment/[Pp]ackages/
 /unity-environment/[Uu]nity[Pp]ackage[Mm]anager/
 /unity-environment/Assets/AssetStoreTools*
+/unity-environment/Assets/Plugins*
 
 # Tensorflow Model Info
 /models