Add Citrix company description to companies.html

Citrix's entry was looking pretty pithy in comparison to the rest, so I updated it to include details about how we use OCaml, and a link to our github page.

Signed-off-by: Mike McClurg <mike.mcclurg@citrix.com>
Merge pull request #114 from mcclurmc/master

Add Citrix company description to companies.html