docs: Document cosa build-fast

I was surprised this was omitted.  Came up in chat.

Author: cgwalters

docs/working.md

@@ -111,18 +111,43 @@ In the future, it's likely coreos-assembler will also support something
 like `overrides/src` which could be a directory of symlinks to local
 git repositories.
 
-## Using cosa run --bind-ro for even faster iteration
+## Using cosa build-fast
 
 If you're working on e.g. the kernel or Ignition (things that go into the initramfs),
 then you probably need a `cosa build` workflow (or `cosa buildinitramfs-fast`, see below).
-However, let's say you want to test a change to something much later in the boot process - e.g. `podman`.  Rather
+However, let's say you want to test a change to something that runs purely in the real root,
+such as e.g. `rpm-ostree`, `podman`, or `console-login-helper-messages`.  Rather
 than doing a full image build each time, a fast way to test out changes is to use
-something like this:
+`cosa build-fast`.
+
+This command assumes you have a previous local coreos-assembler build.  From
+the git checkout of the project you want to add:
+
+```
+$ export COSA_DIR=/srv/builds/fcos
+$ cosa build-fast
+$ cosa run
+```
+
+The `cosa build-fast` command will run `make` and inject the resulting binaries
+on a qcow2 overlay file, which will appear in your project working directory.
+The `cosa run` command similarly knows to look for these `qcow2` files.
+
+This will not affect the "real" cosa build in `/srv/builds/fcos`, but will
+use it as a data source.
+
+## Using cosa run --bind-ro for even faster iteration
+
+This workflow can be used alongside `cosa build-fast`, or separate from it.
+If you invoke e.g.
 
 ```
 $ cosa run --bind-ro ~/src/github/containers/podman,/run/workdir
 ```
 
+The target VM will have the source directory bound in `/run/workdir`; then you
+can directly copy binaries from your development environment into the VM.
+
 If you are running cosa in a container, you will have to change your current
 working directory to a parent directory common to both project directories and
 use relative paths: