Add migration guide

Author: stephencelis

Sources/StructuredQueriesCore/Documentation.docc/Articles/MigrationGuides.md

@@ -14,4 +14,5 @@ these guides contain tips to do so.
 
 ## Topics
 
+- <doc:MigratingTo0.18>
 - <doc:MigratingTo0.16>

Sources/StructuredQueriesCore/Documentation.docc/Articles/MigrationGuides/MigratingTo0.18.md

@@ -0,0 +1,18 @@
+# Migrating to 0.18
+
+StructuredQueries 0.18 migrates existing functionality to a new StructuredQueriesSQLite module.
+
+## Overview
+
+StructuredQueries recently introduced a new module, StructuredQueriesSQLite, to house its
+SQLite-specific helpers, and in 0.18 it has migrated many of its existing SQLite helpers into this
+module.
+
+If you are using SharingGRDB this migration should be transparent, but if you are using
+StructuredQueries directly and need access to these helpers, you must now explicitly import this
+helper module:
+
+```diff
+ import StructuredQueries
++import StructuredQueriesSQLite
+```