message: docs/quickstart: add COSA `build-fast` instructions
diff: @@ -75,3 +75,11 @@ cosa build
  ```
  For more details, see `coreos-assembler` [overrides documentation](https://coreos.github.io/coreos-assembler/working/#using-overrides).
  +
  +### `build-fast` for faster iteration
  +
  +It is possible to use the CoreOS Assembler's [`build-fast`][build-fast-cmd] command for faster iteration.
  +See [here][build-fast-instructions] for instructions on fast-building a qemu image for testing.
  +
  +[build-fast-cmd]: https://github.com/coreos/coreos-assembler/blob/master/src/cmd-build-fast
  +[build-fast-instructions]: https://github.com/coreos/coreos-assembler/blob/2f834d37353ca5f40b460eae2aea73ef995bc710/docs/kola/external-tests.md#fast-build-and-iteration-on-your-projects-tests
  