docs: Doc tweak (headers)

Author: Alan Christie

README.md

@@ -181,10 +181,10 @@ RabbitMQ Topic Queue.
 Version 2 uses the `rstream` package and relies on a RabbitMQ stream.
 
 Version 2 also extends messages prior to forwarding them to their socket clients.
-It does this by appending the messages's **offset** in the stream (an `integer`)
-and the message's **timestamp** (also an `integer`), both of which are provided by
-the backing RabbitMQ stream as the messages are received by the Event Stream's consumer.
-The **offset** can be used as a unique message identifier.
+It does this by appending the messages's **offset** in the stream (an `integer`,
+which we refer to as an **ordinal**)), and the message's **timestamp** (also an `integer`),
+both of which are provided by the backing RabbitMQ stream as the messages are received
+by the Event Stream's consumer. The **offset** can be used as a unique message identifier.
 
 When received as a protobuf string the values are appended to the end of the original
 message using `|` as a delimiter. Here is an example, with the message split at the
@@ -209,6 +209,33 @@ for clarity: -
         }
     }
 
+## Connecting to sockets (historical events)
+The streaming service keeps historical events based on a maximum age and file size.
+Consequently you can connect to your websocket and retrieve these historical events
+as long as they remain in the backend streaming queue. You identify the
+start-time of your events by using **headers** in the websocket request for your
+stream's **LOCATION**.
+
+If you do not provide any header value your socket will only deliver new events.
+
+You can select the start of your events buy providing either an **ordinal**,
+**timestamp**, or **datetime** string.
+
+To stream from a specific **ordinal** (**offset**), provide it as the numerical value
+of the header property `X-StreamOffsetFromOrdinal`.
+
+To stream from a specific **timestamp**, provide it as the numerical value
+of the header property `X-StreamOffsetFromTimestamp`.
+
+To stream from a specific **datetime**, provide the date/time string as the value
+of the header property `X-StreamOffsetFromDatetime`. The datetime string is extremely
+flexible and is interpreted by the **python-dateutil** package's `parse` function.
+UTC is used as the reference for messages and the string will be interpreted as a
+UTC value if it has no timezone specification. For example, if you are in CEST and
+it is `13:33` and you want to retrieve times from 13:33 (local time) then you will need
+to provide a **datetime** string value that has the time set to
+`11:33` (the UTC time for 13:33 CEST) or specify `13:33+02:00`
+
 ---
 
 [black]: https://black.readthedocs.io/en/stable