Add explicit documentation for Google storage `token_file` setting

[teddywing]

version 0.14.1
Python 3.5.0
OS X

When trying to configure vdirsyncer for the first time using the google_calendar storage, I didn't set the token_file setting because I couldn't figure out what its value was meant to be.

I ended up doing an OAuth handshake manually to get an access token, which worked at first, until the token expired some time later.

After looking through the code, I realised that vdirsyncer actually does do the OAuth dance, but it was completely not obvious to me beforehand that I should just set token_file to an empty file and have the program do the work.

Can some extra documentation be added to 12c22f1#diff-fd40cf2be7711772de9d8316da038cceR136 (or to the program itself) to clearly explain what to set token_file to?

[untitaker]

Sure! I think it would make sense to append this to the prose at https://github.com/pimutils/vdirsyncer/blob/cc8fe7f7737598d7a63cb4887c5a31b55d7975b4/docs/config.rst#google.

[teddywing]

Cool, thanks! I'll plan to draft a pull request this weekend.

[untitaker]

Great!

[untitaker]

Hey @teddywing any update on this?

[teddywing]

Sorry I never got back to you on this. I've been a little overwhelmed with work and life recently. Thanks for picking this up and adding the docs!