Add an overarching README for the CCNA labs.

lab-topologies/ccna/README.md

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+# CCNA Exam Scenarios
+
+In this directory you will find CML sample labs with a twist.  Each of these correspond to tasks one will see
+in the [CCNA Exam Topics](https://learningnetwork.cisco.com/s/ccna-exam-topics) list.  Each one of these
+labs comes in two forms: an initial scenario, and a solution.
+
+In the Lab Notes of the initial scenario, you will find an exercise or set of exercises to complete around the
+respective CCNA exam topic.  If you get stuck or want to see an example solution to the exercise(s) the solution
+lab will be the same topology with the devices configured correctly.
+
+**Good luck on your studies!**
+