Add protobuf example & README documentation

[grayside]

Problem

We now support receiving events in binary protobuf format, and expect Firestore events to be primarily used in protobuf. Current examples only show JSON parsing.

Proposal

Add examples of how to parse protobuf event payloads with the library in two forms:

• Standalone
• In conjunction with CloudEvent SDK for Go: https://github.com/cloudevents/sdk-go

Rough Example (Untested)

import (
    "log"
    cloudevent "github.com/cloudevents/sdk-go/v2"
    _ "github.com/cloudevents/sdk-go/binding/format/protobuf/v2"
    "github.com/googleapis/google-cloudevents-go/cloud/firestoredata"
)

func handler(w http.ResponseWriter, r *http.Request) {
    event := cloudevent.NewEventFromHTTPRequest(r)
    var target firestoredata.DocumentEventData
    if err := event.DataAs(&target); err != nil {
        log.Fatal("event.DataAs: %v", err)
    }
    fmt.Println(target.Value.Name)
}