Add Timeline documentation

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+# Timeline
+
+Given the complex nature of the Timeline design, it deserves some explanation of
+the design. This document aims to describe how the Elm SDK designs the Timeline,
+so that other projects may learn from it.
+
+## API endpoint disambiguations
+
+Generally speaking, there are a few API endpoints with similar design:
+
+- The [`/sync` endpoint](https://spec.matrix.org/v1.9/client-server-api/#get_matrixclientv3sync),
+which gets the events that the homeserver received most recently.
+- The [`/messages` endpoint](https://spec.matrix.org/v1.9/client-server-api/#get_matrixclientv3roomsroomidmembers),
+which gets any events in the topological order.
+
+As noted in the Matrix spec:
+
+> Events are ordered in this API according to the arrival time of the event on
+> the homeserver. This can conflict with other APIs which order events based on
+> their partial ordering in the event graph. This can result in duplicate events
+> being received (once per distinct API called). Clients SHOULD de-duplicate
+> events based on the event ID when this happens. 
+
+For this reason, the Elm SDK maintains **two independent timelines** that are tied
+together when necessary to form a coherent timeline.
+
+## Elm design
+
+For those unfamiliar, the Elm Architecture breaks into three parts:
+
+- **Model** - the state of the application
+- **View** - a way to turn your state into meaningful information
+- **Update** - a way to update your state based on the Matrix API
+
+Since these concepts are compartmentalized, it is impossible to make an API call
+while executing the **view** function; the Elm SDK must at all times find a way
+to represent its state.
+
+## Timeline
+
+Concerning the Matrix timeline, it is meant to create a representation
+(**Model**) of the timeline, find a way to represent (**View**) it, and find a
+simple way to adjust it with every incoming Matrix API result. (**Update**)
+
+First, we define what a timeline batch is.
+
+### Timeline batch
+
+A timeline batch is something that most Matrix API endpoints return. It is a
+little piece of the timeline and contains the following four pieces of
+information:
+
+1. A list of events that are part of the timeline.
+2. A Filter for which all provided events meet the criteria.
+3. An end batch token that functions as an identifier.
+4. _(Optional.)_ A start token. If not provided, it indicates the start of the
+    timeline.
+
+Here's an example of such a timeline batch:
+
+```
+       |-->[■]->[■]->[●]->[■]->[■]->[●]-->|
+       |                                  |
+       |<---   filter: only ■ and ●   --->|
+       |                                  |
+     start:                              end:
+    <token_1>                          <token_2>
+```
+
+When the Matrix API later returns a batch token that starts with `<token_2>`,
+we know that we can connect it to the batch above and make a longer list of
+events!
+
+At first, this seems quite simple to connect, but there are some difficulties
+that come up along the way.
+
+### Challenge 1: different filters, different locations
+
+When two timeline batches have different filters, we do not know their
+respective location. For example, the following two timeline batches COULD
+overlap, but it is also possible they don't:
+
+```
+       |-->[■]->[■]->[●]->[■]->[■]->[●]-->|
+       |                                  |
+       |<---   filter: only ■ and ●   --->|
+       |                                  |
+     start:                              end:
+    <token_1>                          <token_2>
+
+
+                |-->[★]->[★]->[★]->[★]-->|
+                |                          |
+                |<--   filter: only ★   -->|
+                |                          |
+              start:                      end:
+            <token_3>                    <token_4>
+```
+
+Realistically, there is currently no way of knowing without making more API
+calls. However, just making more API calls isn't a solution in Elm because of
+its architecture.
+
+> **SOLUTION:** As described in the **View** function, we may assume that
+overlapping timeline batches have overlapping events. If they overlap yet have
+no overlapping events, then their filters must be disjoint. If the filters are
+disjoint, we do not care whether they're overlapping.
+