style: add note about alpha protos (#576)
style: add note about alpha protos (#576)

Signed-off-by: Kuat Yessenov <78a0b7007f2d7b7ec75106f37841ba73f23168e2@google.com>

jrajahalme/envoy-api,lyft/envoy-api,lyft/envoy-api,jrajahalme/envoy-api,jrajahalme/envoy-api,jrajahalme/envoy-api,lyft/envoy-api