contributing: update formatting and testing sections
1. Make changes, commit to your fork.
1. Send a pull request with your changes.
+# Formatting
+
+All code must be formatted with `gofmt` (with the latest Go version) and pass
+`go vet`.
+
# Testing
+Tests are required for all samples. When writing a pull request, be sure to
+write and run the tests in any modified directories.
+
## Running system tests
1. Set the `GOLANG_SAMPLES_PROJECT_ID` environment variable to a suitable test project.
`go test github.com/GoogleCloudPlatform/golang-samples/...`
+Note: You may want to `cd` to the directory you're modifying and run
+`go test -v ./...` to avoid running every test in the repo.
+
## Contributor License Agreements
Before we can accept your pull requests you'll need to sign a Contributor
