.gitignore: add policy for new entries

Change-Id: Ib3592d0ed265ca75703eb2dceb9f26a6dbecfcb7
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/189197
Reviewed-by: Damien Neil <dneil@google.com>

.gitignore

@@ -5,3 +5,8 @@ cmd/protoc-gen-go/protoc-gen-go
 cmd/protoc-gen-go/testdata/go.sum
 cmd/protoc-gen-go-grpc/protoc-gen-go-grpc
 cmd/protoc-gen-go-grpc/testdata/go.sum
+
+# This file includes artifacts of the system test 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.