Add task to generate diagram

Added by the gem's generator:

```
bundle exec rails g erd:install
```

We can run this task to generate an entity-relationship diagram e.g.

```
bundle exec rails db:migrate \
	indirect=true \
	inheritance=true \
	polymorphism=true \
	title="Complete (Ruby)" \
	filename="doc/complete-auto-generated-entity-relationship-diagram"
```

See https://voormedia.github.io/rails-erd/customise.html for available
options.

We include the generated diagram in PDF form in our `doc` directory.

lib/tasks/auto_generate_diagram.rake

@@ -0,0 +1,8 @@
+# NOTE: only doing this in development as some production environments (Heroku)
+# NOTE: are sensitive to local FS writes, and besides -- it's just not proper
+# NOTE: to have a dev-mode tool do its thing in production.
+# :nocov:
+if Rails.env.development?
+  RailsERD.load_tasks
+end
+# :nocov: