Better document the getting started guide for local backends

[jaxxstorm]

Problem description

We currently don't have great documentation for when a user gets started with a local backend (ie, pulumi login --local. The CLI outputs some information, but we should document:

• the implication of using a local backend (ie, it can't be moved off that user's machine)
• setting a passphrase (PULUMI_CONFIG_PASSPHRASE)
• where the backend data is stored
• other things I can't think of

I think this needs to go here and just get fleshed out a little more