Update README.md (fixes styled-components#125) (styled-components#131)

* Update README.md (fixes styled-components#125)

Added an FAQ section that outlines why Jest Styled Components might not be working.

* Update README.md (fixes styled-components#125)

README.md

@@ -45,6 +45,7 @@ Table of Contents
       * [Preact](#preact)
    * [toHaveStyleRule](#tohavestylerule)
    * [Global installation](#global-installation)
+   * [Working with multiple packages](#working-with-multiple-packages)
    * [Contributing](#contributing)
 
 # Snapshot Testing
@@ -384,6 +385,12 @@ And import the library once in the `setupTest.js` as follows:
 import 'jest-styled-components'
 ```
 
+# Working with multiple packages
+
+If Jest Styled Components is not working, it is likely caused by loading multiple instances of `styled-components`. This can happen especially when working with a Lerna monorepo. Starting with `styled-components@3.2.0`, a warning will be logged when multiple instances of it are being included and run as part of the Jest tests. Using `styled-components@3.1.6` and lower with multiple instances will cause a silent error with unexpected results.
+
+To debug and fix multiple instances of `styled-components` see the FAQ on ["Why am I getting a warning about several instances of module on the page?"](https://www.styled-components.com/docs/faqs#why-am-i-getting-a-warning-about-several-instances-of-module-on-the-page).
+
 # Contributing
 
 Please [open an issue](https://github.com/styled-components/jest-styled-components/issues/new) and discuss with us before submitting a PR.