README.md: clarify that Android is for client-side use (#4408)
README.md: clarify that Android is for client-side use (#4408)


carl-mastrangelo/grpc-java,grpc/grpc-java,elandau/grpc-java,stanley-cheung/grpc-java,zhangkun83/grpc-java,stanley-cheung/grpc-java,dapengzhang0/grpc-java,carl-mastrangelo/grpc-java,dapengzhang0/grpc-java,zhangkun83/grpc-java,carl-mastrangelo/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,grpc/grpc-java,elandau/grpc-java,dapengzhang0/grpc-java,grpc/grpc-java,elandau/grpc-java,zhangkun83/grpc-java,zhangkun83/grpc-java,elandau/grpc-java,ejona86/grpc-java,ejona86/grpc-java,stanley-cheung/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,ejona86/grpc-java,carl-mastrangelo/grpc-java