CONTRIBUTORS: add Alexis Imperial-Legrand (Google CLA)

R=golang-dev, bradfitz
CC=ail, golang-dev
https://golang.org/cl/13441050

CONTRIBUTORS

@@ -53,6 +53,7 @@ Alexandre Normand <alexandre.normand@gmail.com>
 Alexandru Moșoi <brtzsnr@gmail.com>
 Alexei Sholik <alcosholik@gmail.com>
 Alexey Borzenkov <snaury@gmail.com>
+Alexis Imperial-Legrand <ail@google.com>
 Amir Mohammad Saied <amir@gluegadget.com>
 Amrut Joshi <amrut.joshi@gmail.com>
 Andrea Spadaccini <spadaccio@google.com>