Add libpng warning to install instructions

README.md

@@ -15,6 +15,18 @@ Image loader module for webpack
 $ npm install image-webpack-loader --save-dev
 ```
 
+### libpng issues
+
+Installing on some versions of OSX may raise errors with a [missing libpng dependency](https://github.com/tcoopman/image-webpack-loader/issues/51#issuecomment-273597313): 
+```
+Module build failed: Error: dyld: Library not loaded: /usr/local/opt/libpng/lib/libpng16.16.dylib
+```
+This can be remedied by installing the newest version of libpng with [homebrew](http://brew.sh/):
+
+```sh
+brew install libpng
+```
+
 ## Usage
 
 [Documentation: Using loaders](http://webpack.github.io/docs/using-loaders.html)
@@ -233,6 +245,7 @@ Default: `false`
 
 Print verbose status messages.
 
+
 ## Inspiration
 
 * [gulp-imagemin](https://github.com/sindresorhus/gulp-imagemin)