Remove a lot of stuff from CONTRIBUTING.
Remove a lot of stuff from CONTRIBUTING.

Some of this was due to old code.google.com workflow.

rouzier/gopacket,google/gopacket,xuzhaokui/gopacket,rouzier/gopacket,rouzier/gopacket,google/gopacket,xuzhaokui/gopacket,google/gopacket,xuzhaokui/gopacket