Add `--watch` command info to README
Would be curious to hear your opinions on implementing a `watch` command. My initial thoughts for the workflow would be:

1. Within an initialized project, run the `skpm watch` command
2. `skpm` would then listen for all changes within the `src` (and `Resources`?) directory. Alternatively the user could set a `watch_glob` (or something similar) within the `package.json`?
3. When a change is detected, it would essentially run the `skpm build` and `skpm link` commands.
4. The changes would now be reflected within Sketch.

My main motivation for this request is to be able to make a change and have that change immediately reflected within Sketch. This seems like a much better workflow compared to running `skpm build` and `skpm link` manually after each change.

Do you have any thoughts or opinions on the above? Does this functionality already exist and I'm just missing it?

Thanks.