leaflet can be installed with opam:
opam install leaflet
If you don't have
opam, you can install it following the how to install opam guide.
If you can't or don't want to use
opam, consult the opam file for build instructions.
Have a look at the example folder. The documentation can be consulted via
odig doc leaflet.
You will also need leaflet's files (CSS and images).
leaflet's original code was taken from a NGI0 funded project