Create a dedicated development & release guide

As discussed in #218, I think we ought to split out the development instructions for Try PureScript from the main README, and we should expand those instructions to include cutting new releases of the project.

The development instructions should cover what they already do: how to set up the local server, how to set up the local client, and how to update the packages and package set.

The release instructions could include things not currently described, including:

• Updating the hardcoded tag used for examples
• How to check that the externs for an upgraded package set don't overwhelm the server
• How to run a deployment

and ideally this could be aggregated in checklist form, so someone who wants to make a release can move step-by-step through the instructions from a fresh clone of the repository.

Start with the main README and the discussion in #218, then trace the existing instructions for setting up the local server and client and updating packages and the package set. Done means a dedicated guide with a fresh-clone checklist covering those development steps plus tag updates, externs checks, and deployment.