Updated Contribution Checklist - Preference for plain text emails.

[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.

I also provide a useful link to a website that shows how to configure a wide
array of clients for plain text, and if the MUA can even be configured as such.

-- 
Cheers,
Carlos.