.gitignore: exclude /.idea/

Some IDEs (e.g. intellij IDE based ones) create the .idea folder
to store project specific settings. This is irrelevant to Go project
that does not assume any specific IDEs, but interferes with git.

Change-Id: I0c93d9a3f7edff095fbe0c7a53b06c92b391c970
Reviewed-on: https://go-review.googlesource.com/53770
Reviewed-by: Ian Lance Taylor <iant@golang.org>

.gitignore

@@ -43,3 +43,6 @@ _testmain.go
 /test/pass.out
 /test/run.out
 /test/times.out
+
+# IntelliJ-based editors
+/.idea/