Update protobuf README.
Update protobuf README.

- Remove mention of non-existent proto compiler
- Add info about updating protobuf.bzl and BUILD files

Fixes https://github.com/bazelbuild/bazel/pull/1769#issuecomment-247966248
Fixes https://github.com/bazelbuild/bazel/issues/1790

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MOS_MIGRATED_REVID=133572313

Asana/bazel,bazelbuild/bazel,akira-baruah/bazel,mrdomino/bazel,dropbox/bazel,snnn/bazel,spxtr/bazel,bazelbuild/bazel,safarmer/bazel,ulfjack/bazel,Asana/bazel,dslomov/bazel-windows,katre/bazel,damienmg/bazel,mbrukman/bazel,perezd/bazel,mrdomino/bazel,ulfjack/bazel,variac/bazel,safarmer/bazel,dslomov/bazel,mbrukman/bazel,ButterflyNetwork/bazel,ButterflyNetwork/bazel,dropbox/bazel,spxtr/bazel,dropbox/bazel,dslomov/bazel,zhexuany/bazel,bazelbuild/bazel,katre/bazel,juhalindfors/bazel-patches,davidzchen/bazel,werkt/bazel,dslomov/bazel-windows,zhexuany/bazel,aehlig/bazel,hermione521/bazel,LuminateWireless/bazel,LuminateWireless/bazel,katre/bazel,safarmer/bazel,twitter-forks/bazel,davidzchen/bazel,dslomov/bazel,aehlig/bazel,juhalindfors/bazel-patches,mrdomino/bazel,mbrukman/bazel,akira-baruah/bazel,kchodorow/bazel,davidzchen/bazel,ButterflyNetwork/bazel,ulfjack/bazel,zhexuany/bazel,spxtr/bazel,Asana/bazel,ButterflyNetwork/bazel,LuminateWireless/bazel,snnn/bazel,snnn/bazel,akira-baruah/bazel,zhexuany/bazel,safarmer/bazel,damienmg/bazel,dslomov/bazel-windows,twitter-forks/bazel,Asana/bazel,cushon/bazel,kchodorow/bazel,kchodorow/bazel,meteorcloudy/bazel,ulfjack/bazel,cushon/bazel,bazelbuild/bazel,meteorcloudy/bazel,damienmg/bazel,variac/bazel,snnn/bazel,davidzchen/bazel,damienmg/bazel,iamthearm/bazel,dslomov/bazel,mikelikespie/bazel,mikelikespie/bazel,mbrukman/bazel,akira-baruah/bazel,cushon/bazel,perezd/bazel,hermione521/bazel,mrdomino/bazel,meteorcloudy/bazel,mikelikespie/bazel,twitter-forks/bazel,kchodorow/bazel,cushon/bazel,spxtr/bazel,kchodorow/bazel,variac/bazel,dropbox/bazel,twitter-forks/bazel,iamthearm/bazel,iamthearm/bazel,iamthearm/bazel,juhalindfors/bazel-patches,meteorcloudy/bazel,twitter-forks/bazel,perezd/bazel,snnn/bazel,davidzchen/bazel,ulfjack/bazel,hermione521/bazel,iamthearm/bazel,damienmg/bazel,meteorcloudy/bazel,dslomov/bazel,snnn/bazel,bazelbuild/bazel,dslomov/bazel-windows,perezd/bazel,twitter-forks/bazel,bazelbuild/bazel,juhalindfors/bazel-patches,variac/bazel,aehlig/bazel,spxtr/bazel,Asana/bazel,mikelikespie/bazel,juhalindfors/bazel-patches,perezd/bazel,ulfjack/bazel,akira-baruah/bazel,katre/bazel,kchodorow/bazel-1,ulfjack/bazel,werkt/bazel,LuminateWireless/bazel,cushon/bazel,perezd/bazel,LuminateWireless/bazel,juhalindfors/bazel-patches,mrdomino/bazel,kchodorow/bazel-1,dslomov/bazel,meteorcloudy/bazel,kchodorow/bazel,damienmg/bazel,damienmg/bazel,kchodorow/bazel-1,kchodorow/bazel,ButterflyNetwork/bazel,variac/bazel,mrdomino/bazel,aehlig/bazel,davidzchen/bazel,mbrukman/bazel,werkt/bazel,spxtr/bazel,kchodorow/bazel-1,variac/bazel,davidzchen/bazel,katre/bazel,dropbox/bazel,meteorcloudy/bazel,dropbox/bazel,mbrukman/bazel,aehlig/bazel,iamthearm/bazel,aehlig/bazel,mikelikespie/bazel,hermione521/bazel,werkt/bazel,LuminateWireless/bazel,mikelikespie/bazel,katre/bazel,dslomov/bazel-windows,zhexuany/bazel,safarmer/bazel,kchodorow/bazel-1,snnn/bazel,ButterflyNetwork/bazel,cushon/bazel,Asana/bazel,Asana/bazel,juhalindfors/bazel-patches,perezd/bazel,zhexuany/bazel,werkt/bazel,akira-baruah/bazel,twitter-forks/bazel,aehlig/bazel,variac/bazel,hermione521/bazel,dslomov/bazel,kchodorow/bazel-1,werkt/bazel,hermione521/bazel,spxtr/bazel,safarmer/bazel,dslomov/bazel-windows