Add sass-watcher script with some hints about how to install it.

.gitignore

@@ -13,4 +13,5 @@ eggs/
 parts/
 src/
 var/
-.sass-cache
+.sass-cache
+/.ruby-version

sass-watcher.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env sh
+
+command -v sass 2>&1 > /dev/null
+has_sass=$?
+
+command -v rbenv 2>&1 > /dev/null
+has_rbenv=$?
+
+
+if [[ $has_sass -eq 1 ]]; then
+    echo 'It seems you do not have "sass" installed yet.'
+
+    if [[ $has_rbenv -eq 0 ]]; then
+        echo "You have rbenv installed."
+        rbenv local 2>&1 > /dev/null
+        rbenv_enabled=$?
+        if [[ $rbenv_enabled -eq 1 ]]; then
+            echo "rbenv is not enabled, enable it with:"
+            echo " $ rbenv local 1.9.3-p194"
+        else
+            echo "Install sass with:"
+            echo " $ gem install sass"
+        fi
+    fi
+
+    exit 1;
+fi
+
+sass --watch plonetheme/onegov/resources/sass/main.scss:plonetheme/onegov/resources/css/main.css