Note on organization of the repo.

README.md

@@ -16,3 +16,24 @@ Development
 ```bash
 $ yarn run dev
 ```
+
+API Packages
+------------
+
+Since it's not entirely clear yet how APIs will develop in future, the approach
+taken here is to package individual APIs up individually. That is, instead of
+providing an overall API version in `api-base.yml`, it should be part of each
+API package's path.
+
+For example, for a `foo` API in its version `v1`, its definitions should live
+in `./paths/foo/v1.js` and `./paths/foo/v1/*.js` respectively.
+
+Interface/implementation
+------------------------
+
+For reusability across API versions, it's best to keep files in the `paths`
+subfolder very thin, and instead inject implementations via the `dependencies`
+configuration value of `express-openapi`.
+
+These implementations line to the `./lib` subfolder. Adjust `server.js` as
+needed to make them available to API packages.