Add more detailed instructions for packagers to change update_check_interval

[Luflosi]

I think this would be very helpful for packagers that don't know the code base.
I'm a total rst noob and don't know how to put the code-block inside the note, maybe you can do that.

[kovidgoyal]

I dont think a patch is a good idea, too much might change in that file. Instead just use something like, find update_check_interval in the config_data.py and change the default value from 24 to 0.

[nightuser]

Maybe it should be set from an environment variable during compilation?

[Luflosi]

Yeah, that's a better solution.