Add README.md for Bazel's third_party/protobuf.

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MOS_MIGRATED_REVID=95640440

third_party/protobuf/README.md

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+How to update these files:
+
+1. Go to http://search.maven.org/
+2. Search for g:"com.google.protobuf"
+3. Download the "jar" link from protobuf-java.
+4. Download all binaries from "protoc".
+5. Strip version number from protoc files: for i in *.exe; do mv $i $(echo $i | sed s/3.0.0-alpha-3-//); done
+6. Set executable bit: chmod +x *.exe
+7. Update third_party/BUILD to point to the new jar file.
+8. Done.
+