Update extensions section to include cuda

[rolfv]

Update MPI extensions section to include CUDA piece

(cherry picked from commit ompi-mpi/ompi@d184f260adddf9fab307adc94f2c8e08853485cf)

[rolfv]

bot:milestone:v2.0.0
bot:assign: @jsquyres

[jsquyres]

@rolfv Do you have a man page for your cuda extension? If so, do you want to cite it in the README? E.g., "For more information, see ....".

[rolfv]

There is one. Maybe we could just go with what we have now and then I will update it later. I think I still want to make some other changes in the CUDA extensions like fix the return value and have them build even when CUDA-aware is not there but just have them set to the right values.

[jsquyres]

@rolfv Sure, sounds fine -- just don't forget (but we're not going to immediately release v2.0.0, so you've got some time).

Can you do me a favor and file an issue against 2.0.0 with what you plan to do for 2.0.0 (i.e., the stuff you mentioned in your comment), so that we can track it? Thanks.