Add installation instructions

README.md

@@ -2,8 +2,10 @@
 A [dynamic Firefox
 theme](https://developer.mozilla.org/en-US/Add-ons/Themes/Theme_concepts#Dynamic_themes)
 that uses the color palette of the current [National Geographic Photo of the Day](https://www.nationalgeographic.com/photography/photo-of-the-day/).
-The theme automatically updates every day, and you can click the toolbar button to
-see the picture and click through to the National Geographic page.
+The theme automatically updates every day, and you can click the toolbar button
+to see the picture and click through to the National Geographic page. Available
+in [Firefox
+Add-ons](https://addons.mozilla.org/en-US/firefox/addon/picture-paint/).
 
 ## Gallery
 ![blue](https://i.imgur.com/hn6DOl4.png)
@@ -14,16 +16,18 @@ see the picture and click through to the National Geographic page.
 ---
 ![red](https://i.imgur.com/JaDy3lV.png)
 
-## Requirements
-* Firefox 60 and above
+## Installation
+This extension requires Firefox 60 and above. Install it from the
+[Add-ons](https://addons.mozilla.org/en-US/firefox/addon/picture-paint/) page.
+It might take a few seconds to process the image and then apply the theme.
 
 ## Build Instructions
 I use the Node version specified in `.nvmrc`. Run `$ yarn install && yarn run
 build:release`. This should generate the final JavaScript files and place
 them in `extension/js`.
 
-To generate a ZIP file for uploading to the [Firefox Add-ons
-site](https://addons.mozilla.org/), run `$ yarn run release`.
+To generate a ZIP file for uploading to [Add-ons](https://addons.mozilla.org/),
+run `$ yarn run release`.
 
 If you have Python 3 and Docker, you can also run `$ ./dev` for a development
 CLI.