Add troubleshooting section to README

[n-riesco]

• FileNotFoundError: [Errno 2] No such file or directory: 'ijskernel' (Ijavascript kernel will not start on jupyter-desktop #259)

• Reinstall (npm uninstall -g ijavascript; npm install -g ijavascript) or build from source (npm_config_zmq_external=true npm install -g ijavascript), when:

Error: The module '/usr/local/lib/node_modules/ijavascript/node_modules/zeromq/build/Release/zmq.node'
was compiled against a different Node.js version using
NODE_MODULE_VERSION 72. This version of Node.js requires
NODE_MODULE_VERSION 83. Please try re-compiling or re-installing
the module (for instance, using `npm rebuild` or `npm install`).

• install with sudo or as root (Sudo install via npm v5 or above (--unsafe-perm) #146)

• For more troubleshooting, see issues labelled as question: https://github.com/n-riesco/ijavascript/issues?q=label%3Aquestion