Added a terminal emulator warning line for using c-tab combo with usnips

Hello,
a little late but creating a pull request as asked in an email.

"I have a proposal to add a warning about not using c-tab combination for
ultisnips triggers. I just lost lots of time debugging this. It works in gvim but
not in vim. It's related to terminal emulators. See:
https://bugs.launchpad.net/ultisnips/+bug/987860

regards,
Ivo "