Added instructions for getting capa sandboxed.

common/lib/capa/capa/safe_exec/README.rst

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+Configuring Capa sandboxed execution
+====================================
+
+Capa problems can contain code authored by the course author.  We need to
+execute that code in a sandbox.  We use CodeJail as the sandboxing facility,
+but it needs to be configured specifically for Capa's use.
+
+As a developer, you don't have to do anything to configure sandboxing if you
+don't want to, and everything will operate properly, you just won't have
+protection on that code.
+
+If you want to configure sandboxing, you're going to use the `README from
+CodeJail`__, with a few customized tweaks.
+
+__ https://github.com/edx/codejail/blob/master/README.rst
+
+
+1. At the instruction to install packages into the sandboxed code, you'll 
+   need to install both `pre-sandbox-requirements.txt` and 
+   `sandbox-requirements.txt`::
+
+    $ sudo pip install -r pre-sandbox-requirements.txt
+    $ sudo pip install -r sandbox-requirements.txt
+
+2. At the instruction to create the AppArmor profile, you'll need a line in
+   the profile for the sandbox packages.  <EDXPLATFORM> is the full path to
+   your edx_platform repo::
+
+    <EDXPLATFORM>/common/lib/sandbox-packages/** r,
+
+That's it.  Once you've finished the CodeJail configuration instructions,
+your course-hosted Python code should be run securely.