Add install instructions

It is not obvious how to install this card with HACS, so this commit documents the process.

In particular, it is good to explicitly mention the type is "plugin", as this can be confusing (eg #1)

README.md

@@ -4,7 +4,32 @@
 
 ![Screenshot of Hourly UV Index Card](screenshot.png)
 
-## Options
+## Install instructions
+
+### Recommended: Install with HACS
+
+1. Ensure you have installed https://github.com/davidn/epa_uvindex first.
+
+2. Navigate to the HACS add-on. Go to "Frontend", if HACS has separate pages for Integrations and Frontend.
+
+2. Via the "..." menu at the top right, select "Custom repositories" and add https://github.com/davidn/uvindex-hourly as type "Plugin".
+
+3. If there is a "Explore & download respositories" button, click on it and find the "Hourly UV Index Card" plugin. If you directly see a list of plugins, find the "Hourly UV Index Card" plugin (you may need to clear filters to see integrations that haven't been downloaded). Click on the plugin and then click "Download".
+
+4. When you are prompted to reload the frontend, do so.
+
+### Manual Install
+
+1. Copy `dist/uvindex-hourly.js` in this repository to `/homeassistant/www/community/uvindex-hourly`.
+
+2. Refresh your browser (Clearing cache).
+
+
+## Usage
+
+Select "Custom: Hourly UV Index Card" when adding a card to the dashboard. Ensure the entity references one from the epa_uvindex integration.
+
+### Options
 
 | Name              | Type    | Requirement  | Description                                 | Default             |
 | ----------------- | ------- | ------------ | ------------------------------------------- | ------------------- |