README: Added config.json steps to Build steps

Author: rdworth

README.md

@@ -13,7 +13,9 @@ The `xmllint` and `xsltproc` utilities need to be in your path. If you are on Wi
 ### Build
 
 1. `npm install`
-2. `grunt`
+2. `cp config-sample.json config.json`
+3. Edit config.json per https://github.com/scottgonzalez/grunt-wordpress#config
+4. `grunt`
 
 ## Style Guidelines