Readme improvements

Updated readme with more helpful setup instructions.

v1.0.2

• Make it clear that dev-console messages are coming from the plugin by prefixing with "[open-in-diffusion]"
• Integrate with busy-signal to give more feedback during startup and while opening a file.

v1.0.1

Dramatically improve plugin activation speed by deferring require() statements, and deferring activation until files or windows are opened.

After initial compilation/caching the activation time should drop from ~200ms to ~2ms.

v1.0.0

Initial release

• Creates Open In Diffusion command that reads from ~/.arcrc to find and query your phabricator/diffusion installation.
• Creates Clear Cache command to reset session history