create src/mvp/15-item-cid.md for dwyl/mvp#410

src/mvp/15-item-cid.md

@@ -0,0 +1,12 @@
+# Use `cid` for `items`
+
+By default the `items` table uses an auto-incrementing (`Serial`) 
+for the `id` (Primary Key):
+
+![items-id-integer](https://github.com/dwyl/mvp/assets/194400/d3020e53-2ad8-43ff-b0ed-95251ee21a87)
+
+This is fine for a _server_-side rendered app 
+with a _single_ relational database instance.
+But our ambition has always been 
+to build a mobile + offline-first distributed App.
+

src/mvp/17-list_items.md

@@ -65,9 +65,11 @@ if we want to avoid conflicts.
 
 As previously noted, the
 `mix phx.gen.schema` command
-does not create any `CRUD` functions 
+does _not_ create any `CRUD` functions 
 or the corresponding `test` file
 for the `schema`.
+In our case that's OK,
+because we want to write a few bespoke functions.
 
 Create the file with the path:
 `test/app/list_items_test.exs`