Add guide to running local acceptance tests.

Point to it in the README_DEVELOPER, and also update the same to refer to beaker rather than the deprecated puppet-acceptance.

Note that this guide actually working is blocking on puppetlabs/beaker#100 shipping in the beaker gem.

@jpartlow Should the guide be incorporated into the `acceptance/README.md`? That claims to address running the tests locally, but when I followed it I was only able to run the tests on VMs in our cloud, rather than on local VMs (reducing its utility for community members without access to our cloud).