Add .iml to ignore target.

Summary:
Sorry for trivial one, but I feel no need to share '.iml' in a project, so it should be ignored.
Closes https://github.com/facebook/react-native/pull/7689

Differential Revision: D3334367

fbshipit-source-id: 2a68aa93084a51d0b3d24427c71bc7c12e41ed78

local-cli/generator/templates/_gitignore

@@ -24,6 +24,7 @@ project.xcworkspace
 
 # Android/IJ
 #
+*.iml
 .idea
 .gradle
 local.properties