Add Docker instructions for bind mounting the synapse module

[th0mcat]

This will help people bind mount the correct directory and get the synapse module working. The only problem I foresee is when synapse updates the version of python it is using, so it would be preferable if synapse contained a plugin directory that we could just bind mount the folder to, but that sounds like a feature request for synapse.

Special thanks to @daedric7 (@daedric:aguiarvieira.pt) for assisting me on figuring this out!

signed-off-by: thomcat [email protected]