docs: appliance calculator docs

Author: davidsauntson

docs/appliance-calculator/README.md

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+# The Appliance Calculator
+
+## Overview
+
+The Appliance Calculator allows users to enter the appliances wish to compare, how long they use them for and how much they pay for
+their electricity. It then provides a table ordered from most to least expensive.
+
+Controllers, views and components have been scoped to the `ApplianceCalculator` namespace.
+
+## Form
+
+The app uses the `wizard_steps` gem to provide the multi-step form functionality to create a `DailyUsage` object. These are stored in the
+session (up to a maximum of 10) and rendered into the `UsagesTableComponent`.
+
+## Interactive Tool Iframe
+
+The app is iframed into an advice page using the `InteractiveToolRenderer` in the public-website repo. A content type exists in Contentful that represents a tool and allows it to be embedded into an advice collection. The renderer in public website then creates and renders an `InteractiveToolComponent`, which is essentially an `iframe` with a `src` that points to this application.
+
+The iframe is resized as instructed by the `postMessage` sent to the `parent` window via the js in the `app/javascript/modules/resize-iframe.js` module.
+
+There are some more notes [interactive tool docs](https://github.com/citizensadvice/public-website/blob/f019add8aac688659f61548afcff727811d281d4/docs/advice/interactive-tools.md?plain=1#L1).
+
+## Contentful Entries
+
+### Appliances
+
+Appliances in the app are represented by the `Appliance` content type. Like intranet content, test and production data are both stored in the `master` Contentful environment. The `test` tag in Contentful is used to distinguish between the two.
+
+Details of the model fields and their meanings can be found in the energy team's Google Drive: https://docs.google.com/document/d/1WhrKpLv7HiOKMuSQ_APuD38Pi6w3ouiY7AvZsjTMQQs/edit?usp=sharing
+
+#### Cyclical & Time Based Appliances
+
+There are two types of appliance: cyclical and time based.  Cyclical appliances have fixed consumptions each time they are used, like a washing machine.  Time based usages have variable consumptions that depend on how long they are used for, like a TV.  The `CyclicalDailyUsage` and `TimeBasedDailyUsage` objects contain the logic for working out `kWh` consumption based on the data entered by the user.