Updated Contribution Checklist - Preference for plain text emails.

[Florian Weimer]

* Carlos O'Donell:

> Developers,
>
> I added a section on "Plain text emails" under "Emailing your patch" here:
> https://sourceware.org/glibc/wiki/Contribution%20checklist#Plain_text_emails
>
> This section states a preference for plain text emails, which avoids problems
> with CI/CD systems or spam detection.

Is it okay to send patches as attachments?  Technically that's MIME as
well and not really “plain text”.  Maybe this should say “non-HTML
email” instead?

Thanks,
Florian