(#5849) Convert rst to markdown (#6040)

Converting some of the rst documentation to markdown.  Attempted to
preserve whitespace and line breaks to minimize cosmetic change.

docs/replication.md

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+# Replication Architecture
+
+## Motivation
+
+We'd like to be able to split some of the work that synapse does into
+multiple python processes. In theory multiple synapse processes could
+share a single postgresql database and we\'d scale up by running more
+synapse processes. However much of synapse assumes that only one process
+is interacting with the database, both for assigning unique identifiers
+when inserting into tables, notifying components about new updates, and
+for invalidating its caches.
+
+So running multiple copies of the current code isn't an option. One way
+to run multiple processes would be to have a single writer process and
+multiple reader processes connected to the same database. In order to do
+this we'd need a way for the reader process to invalidate its in-memory
+caches when an update happens on the writer. One way to do this is for
+the writer to present an append-only log of updates which the readers
+can consume to invalidate their caches and to push updates to listening
+clients or pushers.
+
+Synapse already stores much of its data as an append-only log so that it
+can correctly respond to `/sync` requests so the amount of code changes
+needed to expose the append-only log to the readers should be fairly
+minimal.
+
+## Architecture
+
+### The Replication Protocol
+
+See [tcp_replication.md](tcp_replication.md)
+
+### The Slaved DataStore
+
+There are read-only version of the synapse storage layer in
+`synapse/replication/slave/storage` that use the response of the
+replication API to invalidate their caches.