Add note that NODE_PATH is not supported

browserify doesn't implicitly set opts.paths to NODE_PATH anymore
unlike stated previously in this documentation. For background see:

* browserify/resolve#39 (comment)
* browserify/resolve#47
* browserify/browserify#1626

readme.markdown

@@ -1293,7 +1293,7 @@ This is because your application is more tightly coupled to a runtime
 environment configuration so there are more moving parts and your application
 will only work when your environment is setup correctly.
 
-node and browserify both support but discourage the use of `$NODE_PATH`.
+browserify supports `opts.paths` (although discourages it), but does not support `$NODE_PATH`.
 
 ## non-javascript assets