Make rsgpr easy to install

[erikmannerfelt]

At the moment, there is no easy way to install rsgpr and no instructions exist for it. I see three things that should be done:

• Add an application section in the flake.nix to be able to install it without fiddling with the source code. (completed in Add flake package (for installing) #20)
• Add instructions for installing it on non-nix machines from source, so it's added to the machine's PATH (completed in Update readme with installation instructions and simple usage #22)
• Potentially publish it to https://crates.io to make it super-simple to install.