added README

README.md

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+# picoGym Practice Challenges
+
+## Layout
+
+Every challenge is located in its own folder.
+The folder name is the challenge id, as found in the URL (<https://play.picoctf.org/practice/challenge/CHALLENGE_ID>)
+
+Inside the folder there is an `inputs` subfolder which just includes all the files from the challenge description.
+
+Executing `main.py` runs the challenge.