docs/dev.md: tell about slim binary builds
Also, tell users who build the slim binary that they need the
lokomotive-kubernetes code on disk.
@@ -13,5 +13,15 @@ cd lokoctl
```bash
cd $GOPATH/src/github.com/kinvolk/lokoctl
-make build
+make
```
+
+To build a "dev" version of lokoctl, use
+
+```
+make update-lk-submodule
+make build-slim
+```
+
+The resulting binary won't include the Lokomotive Kubernetes assets and
+requires the lokomotive-kubernetes code in the submodule directory.
