docs: discuss caching personal access tokens

docs: discuss caching personal access tokens

Describe problems storing personal access tokens in git-credential-cache
and suggest alternatives.

Research suggests that many users are confused about this:

> the point of passwords is that (ideally) you memorise them [so]
> they're never stored anywhere in plain text. Yet GitHub's personal
> access token system seems to basically force you to store the token in
> plain text?

https://stackoverflow.com/questions/46645843/where-to-store-my-git-personal-access-token#comment89963004_46645843
Signed-off-by: M Hickford <mirth.hickford@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>