Add .gitignore and create installation instructions

This pull request primarily does two things:
* Completes a `.gitignore` for active development by ignoring `minotar` (compiled executable), `/pkg/` (golang OS-specific files), and `/src/` (used for holding the dependencies locally).
* Completes the step by step installation instructions on the README.md. This way, it makes it easier and simpler for server owners to host their own Minotar version and reduce the stress on the primary server/s.

Enjoy. :+1: