docs: document installation requirements

Author: gajus

README.md

@@ -18,6 +18,7 @@ In contrast to [`react-css-modules`](https://github.com/gajus/react-css-modules)
   * [Anonymous reference](#anonymous-reference)
   * [Named reference](#named-reference)
 * [Configuration](#configuration)
+* [Installation](#installation)
 * [Example transpilations](#example-transpilations)
   * [Anonymous `styleName` resolution](#anonymous-stylename-resolution)
   * [Named `styleName` resolution](#named-stylename-resolution)
@@ -87,6 +88,14 @@ NODE_ENV=production ./test
 
 Missing a configuration? [Raise an issue](https://github.com/gajus/babel-plugin-react-css-modules/issues/new?title=New%20configuration:).
 
+## Installation
+
+When `babel-plugin-react-css-modules` cannot resolve CSS module at a compile time, it imports a helper function (read [Runtime `styleName` resolution](#runtime-stylename-resolution)). Therefore, you must install `babel-plugin-react-css-modules` as a direct dependency of the project.
+
+```bash
+npm install babel-plugin-react-css-modules --save
+```
+
 ## Conventions
 
 ### Anonymous reference