Add Nix+Nixpkgs to the Building section of the README

[toastal]

The Nixpkgs has been merged for a while. The Nix language is a pure ML dialect too!

Letting users know they can get & use it would be helpful. @ratsclub maybe you have an idea on to how best word it since there’s that tricky bit with the stdlib?

[minoki]

I've added make install target and the installed lunarml command works without passing -B (a wrapper script is installed in $PREFIX/bin). I hope this change makes packaging easier.

Two notes:

• The destination path could be specified with PREFIX variable: make install PREFIX=$out
• make install automatically runs make -C thirdparty install