Dub use local fork

[Nicholas Wilson]

I want to use a fork of one of my dub dependencies so I can make 
sure that it works before I merge the fork into upstream.

http://code.dlang.org/advanced_usage
says

     Path-based dependencies
     Package descriptions in the dub.json/dub.sdl can specify a 
path instead of a version; this can be used with Git submodules 
or subtrees, or with a known directory layout, to use arbitrarily 
defined versions of a dependency. Note that this should only be 
used for non-public packages.
     Path-based selections
     You can specify arbitrary versions, branches, and paths in 
the dub.selections.json file, even if they contradict the 
dependency specification of the packages involved (note that DUB 
will output a warning in that case).

but doesn't give any examples.

my dub.selections.json is currently:

{
	"fileVersion": 1,
	"versions": {
		"derelict-cl": "2.0.0",
		"derelict-cuda": "2.0.1",
		"derelict-util": "2.1.0",
		"taggedalgebraic": "0.10.7"
	}
}

I want derelict-cl to use
C:\Users\me\Documents\GitHub\DerelictCL

How do I do that?

Thanks
Nic