re-add accidentally deleted file

gax/.gitignore

@@ -0,0 +1,25 @@
+coverage
+npm-debug.log
+**/*.log
+**/node_modules
+.coverage
+.nyc_output
+docs/
+protos/google/
+out/
+system-test/secrets.js
+system-test/*key.json
+build
+.vscode
+package-lock.json
+.system-test-run/
+.kitchen-sink/
+.showcase-server-dir/
+.compileProtos-test/
+.minify-test/
+__pycache__
+doc/
+dist/
+*.tgz
+**/*.tgz
+test/showcase-echo-client/protos/protos.*