Add missing „get” method and GA notice to docs

Author: BodoMinea

README.md

@@ -317,7 +317,7 @@ var options = {
   key: "/VirtualBox/GuestInfo/Net/0/V4/IP"
 }
 
-virtualbox.guestproperty(function guestproperty_callback(machines, error) {
+virtualbox.guestproperty.get(function guestproperty_callback(machines, error) {
   if (error) throw error;
   // Act on machines
 });
@@ -352,6 +352,8 @@ virtualbox.extradata.set(options, function extradataset_callback(error) {
 });
 ```
 
+_Note: some properties are only available/effective if the Guest OS has the (https://www.virtualbox.org/manual/ch04.html)[Guest Additions] installed and running._
+
 # Putting it all together
 
 ```javascript