Update MAINTAINERS.txt.

Closes #3187239

As per xjm: "The correct formatting is to add the name to the list above, with the word (provisional) in front of it. See the PHPUnit Initiative section for an example."

Not clear why the Provisional membership section is left empty on other areas.

core/MAINTAINERS.txt

@@ -551,5 +551,6 @@ participate in mentoring.
 - Elli Ludwigson 'ekl1773' https://www.drupal.org/u/ekl1773
 - Jess Myrbo 'xjm' https://www.drupal.org/u/xjm
 - Matthew Radcliffe 'mradcliffe' https://www.drupal.org/u/mradcliffe
+- (provisional) Chris Darke 'chrisdarke' https://www.drupal.org/u/chrisdarke
 
 Provisional membership: None at this time.