Add basic docs for using dep

docs/dep.md

@@ -0,0 +1,26 @@
+# kubernetes/test-infra dependency management
+
+test-infra uses [`dep`](https://github.com/golang/dep) for Go dependency
+management. `dep` is a prototype dependency management tool for Go. It requires
+Go 1.8 or newer to compile.
+
+
+## Setup
+
+You can follow the [setup instructions](https://github.com/golang/dep#setup) to
+set up `dep` in your local environment.
+
+
+## Changing dependencies
+
+You can use the `dep` instructions for [adding](https://github.com/golang/dep#adding-a-dependency),
+[updating](https://github.com/golang/dep#updating-dependencies) or
+[removing](https://github.com/golang/dep#removing-dependencies) a dependency.
+
+Once you've updated, make sure to run:
+```
+dep prune
+verify/update-bazel.sh
+```
+
+To prune unneeded deps, and then update all the bazel files that `dep` blows away.