Improve GitHub documentation

[dregad]

The current documentation has not been revised since its creation back in 2016.

Moreover, it does not provide any instructions on how to automate the processing of commits as they are pushed to GitHub (previously with Services, and now with Webhooks).

[morgant]

@dregad Good point, I was actually using a cron job to run the update periodically. Thanks for all your work on these updates!

[dregad]

@morgant would you kindly review the updated doc and test the new Github integration functionality in https://github.com/dregad/source-integration/tree/gh-doc ? Let me know your thoughts and if you find any issues.