README: detail how to use bazel rule [skip ci]

Signed-off-by: Chris Koch <chrisko@google.com>

README.md

@@ -59,12 +59,56 @@ git clone https://github.com/gokrazy/gokrazy
 makebb ./p9/cmd/* ./gokrazy/cmd/*
 ```
 
+For the moment, `makebb` is tested with repositories on the local file system.
+Using Go import paths is supported, as well, but not as well-tested.
+
 ### APIs
 
 Besides the makebb CLI command, there is a
 [Go API at src/pkg/bb](https://pkg.go.dev/github.com/u-root/gobusybox/src/pkg/bb)
 and bazel rules in [src/gobb2.bzl](src/gobb2.bzl).
 
+#### Using bazel go_busybox rule
+
+Assuming you have [rules_go](https://github.com/bazelbuild/rules_go) set up, add
+the following to your `WORKSPACE`:
+
+```bzl
+git_repository(
+    name = "com_github_u_root_gobusybox",
+
+    # We do not have regular releases yet.
+    #
+    # We also do not guarantee compatibility yet, so it may be worth choosing a
+    # commit and setting `commit = "hash"` here instead of the branch.
+    branch = "main",
+    remote = "https://github.com/u-root/gobusybox.git",
+)
+```
+
+Then, in any `BUILD` file, you can create a busybox like this:
+
+```bzl
+load("@com_github_u_root_gobusybox//src:gobb2.bzl", "go_busybox")
+
+go_busybox(
+    name = "bb",
+    cmds = [
+        # These must be absolute labels, for the moment, and each command must
+        # be listed individually. (No :... or :all target patterns.)
+        "//cmd/foobar",
+        "//cmd/otherbar",
+
+        # Another repository's go_binarys are totally fine, e.g. if imported
+        # with gazelle's go_repository rule.
+        "@com_github_u-root_u-root//cmds/core/ls",
+    ],
+)
+```
+
+For the moment, the targets listed on `cmds` must be **individual, absolute
+labels** (issue [#38](https://github.com/u-root/gobusybox/issues/38)).
+
 ### Shortcomings
 
 -   Any *imported* packages' `init` functions are run for *every* command.