Hi JD,
It doesn't look like there's currently much documentation specific to Admin Tool plugins, but they're one of the more generic plugin types, so for the basic structure you should be able refer to the "Standard plugin features" section of the Local Plugin documentation (Local plugins are completely generic with no special APIs) and just replace everywhere you see "/local/" with "/admin/tool/".
If you have anything specific that you're having trouble with, let us know what you've tried and we'll see if we can help!