How to use a Github Token in .netrc file

[afiune]

What does this do / why do we need it?

Adds documentation about a second option to interact with
private repositories, in particular for those who wants to use
a Github Token either locally or in a CI System.

What should your reviewer look out for in this PR?

Grammar or misspelled words.

Which issue(s) does this PR fix?

It is not fixing any issues but it might help others in particular with these issues:

• Support for private repositories e.g. Github Enterprise #174
• Support for private/enterprise patterns #286

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[afiune]

I signed it!

[googlebot]

CLAs look good, thanks!

[sdboyer]

oh cool, yes, .netrc! i knew i'd set something like this up in the past, but i'd entirely forgotten how :)

thank you thank you thank you, doc contributions are amazing! 🎉