[Docs] add PYNQ bitstring package to reqs

docs/finn/getting_started.rst

@@ -18,6 +18,7 @@ Requirements
 * A working Vivado 2019.1 installation
 * A `VIVADO_PATH` environment variable pointing to the Vivado installation directory (e.g. the directory where settings64.sh is located)
 * (optional) A PYNQ board with a network connection
+   * the ``bitstring`` package must be installed on the PYNQ: ``sudo pip install bitstring``
 
 Running FINN in Docker
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