Add a bit of explanation about how the challenge works

docs/medium.md

@@ -16,7 +16,8 @@
 ### Your mission
 1. Hack into the server
 2. Read the hidden password (format=`flag{xxx}`)
-3. <input id="johnFlag"><button onClick="check('johndoor', 'johnFlag')">Check!</button>
+3. Fill in your flag below and check if it is right!
+4. <input id="johnFlag"><button onClick="check('johndoor', 'johnFlag')">Check!</button>
 
 
 ## Finally