.gitignore: include only Go project artifiacts

This effectively reverts

https://golang.org/cl/53770

and adds a guide on what not to add in this file.

Update #21458

Change-Id: I7c740d492b70628b5d9f9e1622014995a3f6f8ec
Reviewed-on: https://go-review.googlesource.com/55871
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: hyangah

.gitignore

@@ -44,5 +44,7 @@ _testmain.go
 /test/run.out
 /test/times.out
 
-# IntelliJ-based editors
-/.idea/
+# This file includes artifacts of Go build that should not be checked in.
+# For files created by specific development environment (e.g. editor),
+# use alternative ways to exclude files from git.
+# For example, set up .git/info/exclude or use a global .gitignore.