docs: mention express as the request/response parameters (#200)
docs: mention express as the request/response parameters (#200)

Signed-off-by: Grant Timmerman <ec5643aa2183a085b989c22c283fa0171e188d54@google.com>
GoogleCloudPlatform/functions-framework-nodejs,GoogleCloudPlatform/functions-framework-nodejs,GoogleCloudPlatform/functions-framework-nodejs