add v6 compatibility note

Jeff Escalante · Jeff Escalante · commit 0b3f9a87dbe2 · 2016-06-07T11:32:43.000-04:00
diff --git a/readme.md b/readme.md
@@ -37,6 +37,8 @@ Spike is fairly strict in enforcing a default stack. However, the stack allows f
 
 ## Installation
 
+> **NOTE:** You must be using >= node v6.0.0 in order for Spike to work!
+
 - `npm install spike -g`
 - `spike new <projectname>`
 
