Add documentation around usage and styles

Author: Tomdango

README.md

@@ -2,7 +2,49 @@
 
 Extensions to the NHS.UK React Components Library
 
-### Storybook
+## Usage
+
+### Installation
+
+The `nhsuk-react-components-extensions` package is published on NPM, and is installable via:
+
+```bash
+yarn add nhsuk-react-components-extensions
+
+# Or
+
+npm i --save nhsuk-react-components-extensions
+```
+
+### Importing Components
+
+Components can either be imported from the global package or from the `/lib` directory. The package is tree-shakeable, so you should not see a sizeable difference between the import sizes using either method.
+
+```jsx
+// From the global package
+import { SubNavigation } from 'nhsuk-react-components-extensions';
+
+// From the lib directory
+import SubNavigation from 'nhsuk-react-components/lib/components/sub-navigation';
+```
+
+### Styles
+
+The package comes with two separate "master" stylesheets. These can be found at `~nhsuk-react-components-extensions/css/all.css` and `~nhsuk-react-components-extensions/css/components.css`.
+
+If you are already using components from `nhsuk-frontend` or the `nhsuk-react-components` packages, it is strongly recommended to use the `components.css` file as this only contains the additional styles required to use the extra components in this library.
+
+If you are not using any of those other packages, or the standard NHS.UK stylesheets, you will require the `all.css` file as this contains all of the core NHS.UK styles as well as the component styles.
+
+```scss
+// Core NHS.UK Styles and Components
+@import '~nhsuk-react-components-extensions/css/all.css';
+
+// Just Components
+@import '~nhsuk-react-components-extensions/css/components.css';
+```
+
+## Storybook
 
 A storybook containing all of the components and their usage can be found [here](https://nhsdigital.github.io/nhsuk-react-components-extensions).