Update readme to mention usage of HTTP headers

[DiscoPYF]

The same way we explain that some methods have an argument to specify serialization optons, I think we should also explain how it works for header values.

Example, below ### Serialization Options:

### HTTP Headers

APIs that support specifying HTTP headers have an optional method argument to pass in header values.

For example, to specify a streaming transaction ID when creating a document:


` ```csharp
await adb.Document.PostDocumentAsync(
    "MyCollection",
    new MyClass
    {
        ItemNumber = 123456,
        Description = "Some item"
    },
    new DocumentHeaderProperties()
    {
        TransactionId = "0123456789"
    });
` ```