ide: Include .idea/zulip.iml

This has some useful `excludeFolder` items that should help Android
Studio see that certain files are not part of our project and should
be excluded in searches that are supposed to be limited to "project
files".

Author: chrisbobbe

.gitignore

@@ -17,6 +17,7 @@ migrate_working_dir/
 /.idea/*
 /android/.idea/*
 !/.idea/inspectionProfiles/
+!/.idea/zulip.iml
 !/android/.idea/inspectionProfiles/
 
 # The .vscode folder contains launch configuration and tasks you configure in