Docs for pre/dev releases

Following up from #147 it would be nice to have some docs for how to setup/use pre and dev releases. I would suggest having the channel be `conda-forge/label/dev_` to match the proposal from https://github.com/conda/conda/issues/8752.

Once this is documented/agreed we can start the bot making dev/pre-release PRs as appropriate. For bot policy discussion see/extend: https://github.com/regro/cf-scripts/issues/86 I think the main concern there is the rate of PRs, maintainer bandwidth, and CI resource availability.