Update the building instructions.
Update the building instructions.

 - GOPATH and GOROOT don't actually need to be empty
 - We don't need the android-27 platform
 - Ask people to install bazel (and msys) before modifying msys
 - Tell people to install bazel on macOS and Linux

Qining/gapid,baldwinn860/gapid,Qining/gapid,google/agi,google/agi,google/gapid,google/gapid,baldwinn860/gapid,google/agi,baldwinn860/gapid,baldwinn860/gapid,pmuetschard/gapid,google/gapid,google/agi,Qining/gapid,pmuetschard/gapid,pmuetschard/gapid,pmuetschard/gapid,google/agi,baldwinn860/gapid,google/agi,google/gapid,google/gapid,Qining/gapid,baldwinn860/gapid,google/gapid,Qining/gapid,google/gapid,google/agi,pmuetschard/gapid,google/gapid,pmuetschard/gapid,Qining/gapid,Qining/gapid,Qining/gapid,baldwinn860/gapid,pmuetschard/gapid,baldwinn860/gapid,google/agi,pmuetschard/gapid