Add local dev instructions to readme

[mdarrik]

This PR adds instructions to and ergonomics to help devs get setup running their locally built Toast binary.

• Adds instructions for installing a locally built binary in the appropriate location
• Adds commands to build and install the binary to the Makefile
• Adds functions for printing the directory the binary gets installed to, and the path to the binary
• Adds snowpack config and type: module to test-toast-site so it can run without incident.