Update README to note that this repo is obsolete?

[artob]

It appears these formulae haven't been updated in a while, and that Homebrew core now has a newer gRPC formula (https://github.com/Homebrew/homebrew-core/blob/master/Formula/grpc.rb).

Perhaps a warning to that effect would be in order for this repository's README, or perhaps the repository ought to be archived?

[Dominik-K]

@stanley-cheung @jtattermusch Can you archive this repository, please? Some people still used this formula to install grpc.

[Dominik-K]

The repositories original purpose is also reached since 2016 (release of protobuf version 3.0:

The formulae will be maintained here rather than adding them to the homebrew or linuxbrew repos until gRPC and protobuf 3.0 are generally available, i.e, gRPC reaches 1.0 and protobuf 3.0 is released.