Update GCE.md
Update GCE.md

Clarified positioning in intro.

Former-commit-id: 814ef224f8b975428f7f40eaa7258fce7a7b47a2
tomdee/libnetwork-plugin,TrimBiggs/libnetwork-plugin,Symmetric/libcalico,TrimBiggs/libcalico,L-MA/libcalico,plwhite/libcalico,tomdee/libcalico,robbrockbank/libcalico,alexhersh/libcalico,TrimBiggs/libnetwork-plugin,projectcalico/libcalico,projectcalico/libnetwork-plugin,djosborne/libcalico,insequent/libcalico,caseydavenport/libcalico