Consider adding a license file

I received this message (from Thomas Toniazzo to Gokhan, forwarded to me):

> One lingering question I have (not sure if someone else has managed
to understand this better) is whether there exists at all a specific
license for CAM as such. Is there?
In the CAM's repo, there is in fact a license file under the
manage_externals directory (it seems standard BSD) but it's not
entirely clear what exactly it covers; is it only for the
manage_externals software itself?

You may want to clear up this situation by putting a LICENSE file in the top level directory.

Here's what we have for CTSM: https://github.com/ESCOMP/CTSM/blob/master/LICENSE

See also http://www.cesm.ucar.edu/models/cesm2/copyright.html