Add notes on upgrading the devbox

Readme.md

@@ -52,3 +52,12 @@ vagrant plugin install vagrant-disksize
 ```
 vagrant up
 ```
+
+# Good to know about this setup aka warnings
+* Default user is vagrant and password is `vagrant`. Feel free to add your own user. 
+* Once the box is up and running, refrain from using the `vagrant box update` feature. 
+Otherwise all your changes might be purged (especially custom user data). 
+Either:
+  * Use the operating system tools to update the box (e.g. apt upgrade) or 
+  * prepare to do a full backup of your data in advance of updating the box 
+  (maybe it's enough to backup the home disk in case there were no changes to the root partition)