[ofiwg] provider feature matrix

[Dave Goodell (dgoodell)]

I've added a new wiki page that attempts to document which providers support which features: https://github.com/ofiwg/libfabric/wiki/Provider-Feature-Matrix

It's deliberately incomplete, as it was difficult in some cases for me to assess the quality of support for non-usnic providers.  If you (the other provider maintainers) can help me fill out the table, I would very much appreciate it.  Also, for the other providers I might have misunderstood the current level of support where I did investigate it, so please correct me if you see something wrong in the tables.

Right now it just documents capabilities, modes, and endpoint types.  But if there's other features/functionality that should be specifically called out, please add to the page.

-Dave