Add fix for pynacl to "Setting up and running Launchpad"

* doc: update running launchpad how-to with pynacl fix for arm64

how-to/running.rst

@@ -265,6 +265,8 @@ Finally, build the database schema (this may take several minutes):
 
     $ make schema
 
+If you get an error while building ``pynacl`` under an ``arm64`` architecture, see :ref:`building-on-arm`.
+
 Running
 =======
 
@@ -417,6 +419,23 @@ the bridge interface on your computer):
     sudo ufw allow in on lxdbr0
     sudo ufw route allow in on lxdbr0
 
+.. _building-on-arm:
+
+Building on ARM
+---------------
+
+When running ``make schema``, installing ``pynacl`` can run into `a bug <https://github.com/pyca/pynacl/issues/553>`_ when building within a virtualized environment on ``arm64`` architecture, leading to ``ERROR: Failed building wheel for pynacl``.
+
+The following needs to be run when having this issue:
+
+.. code-block:: shell-session
+
+    $ sudo apt install --yes libsodium-dev 
+    $ source env/bin/activate
+    $ SODIUM_INSTALL=system pip install --no-binary :all: pynacl
+
+Then run `make schema` again.
+
 Email
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