adds a family tree sftp exersize

modules/resources/exercise_sftp_family.md

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+# One big family exercise
+
+Your larger family has decided you're in charge of hosting a NAS for everybody on both sides of the family.
+In my case my dad has one brother and two sisters an my mum has just two brothers.
+A *family* tree would look like this.
+
+![family tree](./family_tree.png)
+
+Some rules I can think of:
+
+* children can't read the files of the adults
+* every family is a group in itself