Guide: router module (#1279)

* Router module initial commit

* Add the router.md itself

* Enqueue only on enact_candidate

* Initialization concerns

* Ignore the snippet

* Update roadmap/implementors-guide/src/runtime/router.md

Co-authored-by: Robert Habermeier <rphmeier@gmail.com>

* Dispatch messages on finalization

* More fixes from review

* Update roadmap/implementors-guide/src/SUMMARY.md

Co-authored-by: Robert Habermeier <rphmeier@gmail.com>

* Adds router to initializer order

* Adds messages.md

Co-authored-by: Robert Habermeier <rphmeier@gmail.com>

polkadot/roadmap/implementors-guide/src/runtime/router.md

@@ -0,0 +1,35 @@
+# Router Module
+
+The Router module is responsible for storing and dispatching Upward and Downward messages from and to parachains respectively. It is intended to later handle the XCMP logic as well.
+
+For each enacted block the `queue_upward_messages` entry-point is called.
+
+## Storage
+
+Storage layout:
+
+```rust,ignore
+/// Messages ready to be dispatched onto the relay chain.
+/// This is subject to `max_upward_queue_count` and
+/// `watermark_queue_size` from `HostConfiguration`.
+RelayDispatchQueues: map ParaId => Vec<UpwardMessage>;
+/// Size of the dispatch queues. Caches sizes of the queues in `RelayDispatchQueue`.
+/// First item in the tuple is the count of messages and second
+/// is the total length (in bytes) of the message payloads.
+RelayDispatchQueueSize: map ParaId => (u32, u32);
+/// The ordered list of `ParaId`s that have a `RelayDispatchQueue` entry.
+NeedsDispatch: Vec<ParaId>;
+```
+
+## Initialization
+
+No initialization routine runs for this module.
+
+## Routines
+
+* `queue_upward_messages(AbridgedCandidateReceipt)`:
+  1. Updates `NeedsDispatch`, and enqueues upward messages into `RelayDispatchQueue` and modifies the respective entry in `RelayDispatchQueueSize`.
+
+## Finalization 
+
+  1. Dispatch queued upward messages from `RelayDispatchQueues` in a FIFO order applying the `config.watermark_upward_queue_size` and `config.max_upward_queue_count` limits.