readme: el capitan upgrade notice

Readme.md

@@ -31,6 +31,8 @@ Fedora | `sudo yum install cairo cairo-devel cairomm-devel libjpeg-turbo-devel p
 Solaris | `pkgin install cairo pkg-config xproto renderproto kbproto xextproto`
 Windows | [Instructions on our wiki](https://github.com/Automattic/node-canvas/wiki/Installation---Windows)
 
+**El Capitan users:** If you have recently updated to El Capitan and are experiencing trouble when compiling, run the following command: `xcode-select --install`. Read more about the problem [on Stack Overflow](http://stackoverflow.com/a/32929012/148072).
+
 ## Screencasts
 
   - [Introduction](http://screenr.com/CTk)