Activate auto-build for readthedocs

[tkf]

@stevengj Can you configure the webhook as mentioned in https://docs.readthedocs.io/en/latest/webhooks.html#github? Or, I'll do it myself if you are OK with adding me to the admin of https://github.com/JuliaPy/pyjulia. This is required for triggering documentation build for each push. Currently, I'm doing this manually.

ref #243

[tkf]

@stevengj ping