[README] Mention dependencies to build Protagonist

Author: kylef

README.md

@@ -17,7 +17,9 @@ The best way to install Protagonist is by using its [NPM package](https://npmjs.
 $ npm install protagonist
 ```
 
-Works smoothly with node.js ~0.8.15, 0.10.x, 0.12.x, 2 to 4 and iojs v1.x.x
+**NOTE:** *Installing Protagonist depends on having Python 2 installed along with a compiler such as GCC, Clang or MSVC.*
+
+Tested with node.js ~0.8.15, 0.10.x, 0.12.x, 2 to 5 and iojs v1.x.x
 
 ## Getting started