fix: Update README with protobuf 4.x release candidate info (#1049)

Add note about protobuf 4.x release candidate version.

Author: diegomarquezp

README.md

@@ -2,6 +2,7 @@
 
 Shared Maven build configuration for Google Cloud Java client libraries.
 This is not intended for library users.
+This is the protobuf 4.x release candidate version of this library.
 
 [![Maven][maven-version-image]][maven-version-link]