doc(readme): installation via "tools package" (#677)

README.md

@@ -75,15 +75,36 @@ Ginkgo is best paired with Gomega.  Learn more about Gomega [here](https://onsi.
 
 Agouti allows you run WebDriver integration tests.  Learn more about Agouti [here](https://agouti.org)
 
-## Set Me Up!
+## Getting Started
 
-You'll need the Go command-line tools. Ginkgo is tested with Go 1.6+, but preferably you should get the latest. Follow the [installation instructions](https://golang.org/doc/install) if you don't have it installed.
+You'll need the Go command-line tools. Follow the [installation instructions](https://golang.org/doc/install) if you don't have it installed.
 
+### Global installation
+To install the Ginkgo command line interface into the `$PATH` (actually to `$GOBIN`):
 ```bash
+go get -u github.com/onsi/ginkgo/ginkgo
+```
+
+### Go module ["tools package"](https://github.com/golang/go/issues/25922):
+Create (or update) a file called `tools/tools.go` with the following contents:
+```go
+// +build tools
+
+package tools
 
-go get -u github.com/onsi/ginkgo/ginkgo  # installs the ginkgo CLI
-go get -u github.com/onsi/gomega/...     # fetches the matcher library
+import (
+	_ "github.com/onsi/ginkgo"
+)
 
+// This file imports packages that are used when running go generate, or used
+// during the development process but not otherwise depended on by built code.
+```
+The Ginkgo command can then be run via `go run github.com/onsi/ginkgo/ginkgo`.
+This approach allows the version of Ginkgo to be maintained under source control for reproducible results,
+and is well suited to automated test pipelines.
+
+### Bootstrapping
+```bash
 cd path/to/package/you/want/to/test
 
 ginkgo bootstrap # set up a new ginkgo suite