discovery: add .idea directory to .gitignore
discovery: add .idea directory to .gitignore

IntelliJ creates a .idea ignore in the repository root, which is now
ignored by git.

Change-Id: Ieda599bbc0865649e91bb4bee82b001d615a1026
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/447072
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite