Add a short example of working with multiple documents in readme

[DiscoPYF]

In the readme, we have short examples for creating and updating a document. Let's also add corresponding examples showing how to create and update multiple documents.

A quick mention may also be enough, e.g. "To create multiple documents, use PostDocumentsAsync".

I opened this because we've had questions about how to do this. We also discussed separately on adding following links on the readme:

• The C# API reference: https://arangodb-community.github.io/
• The library provides access to ArangoDB HTTP API, so usually it will support whatever the HTTP API supports.
  e.g. documentation for document API: https://www.arangodb.com/docs/stable/http/document-working-with-documents.html