Document how to add Go dependencies

BAZEL-go.md

@@ -0,0 +1,20 @@
+# Working with Go in Bazel
+
+## Dependencies
+
+Go dependencies are managed by [`go.mod`][go-deps] and [Gazelle][gazelle].
+To add a new Go dependency
+
+1. Run `go get` to add the dependency to `go.mod`. E.g.
+    ```
+    go get github.com/envoyproxy/[email protected]
+    ```
+    This should update `go.mod` and `go.sum`.
+2. Run Gazelle to import the dependencies to Bazel.
+    ```
+    bazel run //:gazelle-update-repos
+    ```
+    This should update `go_deps.bzl`.
+
+[go-deps]: https://go.dev/doc/modules/managing-dependencies
+[gazelle]: https://github.com/bazelbuild/bazel-gazelle