Lots of new awesome documentation

docs/backends/sql.md

@@ -0,0 +1,23 @@
+# SQL Backend
+## Configuration
+To be completed. For now, see default structure and values:
+```
+sql:
+  enabled: false
+  type: 'sqlite' or 'postgresql'
+  connection: ''
+  auth:
+    enabled: false
+  directory:
+    enabled: false
+    query:
+      name:
+        type: 'localpart'
+        value: 'SELECT 1'
+      threepid:
+        type: 'localpart'
+        value: 'SELECT 1'
+  identity:
+    type: 'localpart'
+    query: 'SELECT user_id AS uid FROM user_threepids WHERE medium = ? AND address = ?'
+```