PolyGerrit: Update docs on install nodejs 6

nodejs 4 is no longer supported in polymer.

See https://github.com/Polymer/web-component-tester/blob/master/CHANGELOG.md#600---2017-05-15

Which is causing failures for ci. Lets update to nodejs 6.

Change-Id: Ie0e314785e5672e9f557e63c76616d33f601fe24
(cherry picked from commit 722987a7ab91cb6848348a1f83ba5068e05e44c3)
diff --git a/polygerrit-ui/README.md b/polygerrit-ui/README.md
index 76979f1..90bc0ed 100644
--- a/polygerrit-ui/README.md
+++ b/polygerrit-ui/README.md
@@ -2,8 +2,10 @@
 
 ## Installing [Node.js](https://nodejs.org/en/download/)
 
+The minimum nodejs version supported is 6.x+
+
 ```sh
-# Debian/Ubuntu
+# Debian experimental
 sudo apt-get install nodejs-legacy
 
 # OS X with Homebrew