Document Haxelib from git with subfolder

Haxelib allows to get a library from git like:
```
haxelib git hamcrest https://github.com/mikestead/hamcrest-haxe.git master src
```
With arguments: library name, git URL, branch name, subfolder (undocumented).

Likewise you should be able to declare it it using `-lib/-L/--library`:
```
-lib hamcrest:git:https://github.com/mikestead/hamcrest-haxe.git#master#src
```
(found the `#src` argument with some guessing)

It's quite useful when your `haxelib.json` isn't at the root of the repository, which can make things cleaner.