Update README with build instructions.
Update README with build instructions.


Former-commit-id: df72b4d5c02f57c9bf7c3ee6946648b943697aff
robbrockbank/libcalico,tomdee/libnetwork-plugin,insequent/libcalico,TrimBiggs/libnetwork-plugin,projectcalico/libnetwork-plugin,caseydavenport/libcalico,tomdee/libcalico,L-MA/libcalico,alexhersh/libcalico,TrimBiggs/libnetwork-plugin,plwhite/libcalico,TrimBiggs/libcalico,projectcalico/libcalico,djosborne/libcalico,Symmetric/libcalico