docs(plus-checkpoint): add checkpoint restrictions - exam conditions

subjects/devops/plus-checkpoint/README.md

@@ -1,5 +1,19 @@
 ## plus
 
+> 🕵️ **Operation Data Centre**
+>
+> [Confidential]
+>
+> You are on a secret mission inside an underground data centre. You are alone, there is no internet access and there is no mobile phone signal.
+>
+> You cannot interact with anyone.
+>
+> You cannot not access the internet - although you can read the mission below.
+>
+> You may access man pages via the command line. This is your only resource.
+>
+> Good luck agent, we're counting on your knowledge.
+
 ### Instructions
 
 In this exercise, you will make a script `plus.sh` that will take two arguments from the command line, add them and output the result.