Doc & Upgrade Supported libwebsocket Versions

[PrometheusPi]

This is the followup issue to #93 suggested by @ax3l. As realized during setup of isaac, newer versions of libwebsocket are not compatible with isaac. Therefore we need to:

• determine the currently working versions
• update the documentation to clearly state the versions currently working (only 2.1 & 2.2?)
• determine the reason, why newer libwebsocket versions (e.g. 3+) do not work with isaac (and fix that)