Add rekt exfil challenge

forensics-rektexfil/README.md

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+* This forensics challenge was worth `300` points.
+* Download the memory image from [https://mega.nz/#!EYZEkBKA!kr5LRlJ8N0S_beuJs238d3Ac2xy597yYI7miV7GXl7A](https://mega.nz/#!EYZEkBKA!kr5LRlJ8N0S_beuJs238d3Ac2xy597yYI7miV7GXl7A) (hosted externally because it's large).
+
+
+## Important info
+
+An employee has been escorted from the building due to frequent breaches of the IT Security Policy and turning up to work under the influence of alcohol. As a senior analyst in the organisation, you've been tasked with investigating memory image of the employee's virtual machine to search for evidence of data exfiltration or suspicious communications.