Add a basic README to synapse.storage

MANIFEST.in

@@ -13,6 +13,7 @@ recursive-include synapse/storage *.sql.postgres
 recursive-include synapse/storage *.sql.sqlite
 recursive-include synapse/storage *.py
 recursive-include synapse/storage *.txt
+recursive-include synapse/storage *.md
 
 recursive-include docs *
 recursive-include scripts *

synapse/storage/README.md

+Storage Layer
+=============
+
+The storage layer is split up into multiple parts to allow Synapse to run
+against different configurations of databases (e.g. single or multiple
+databases). The `data_stores` are classes that talk directly to a single
+database and have associated schemas, background updates, etc. On top of those
+there are (or will be) classes that provide high level interfaces that combine
+calls to multiple `data_stores`.
+
+There are also schemas that get applied to every database, regardless of the
+data stores associated with them (e.g. the schema version tables), which are
+stored in `synapse.storage.schema`.