Update index.md

Proposing to add GitHub's Green Software Directory to the list as a collection of green software tools available on GitHub.

Author: abbitude

collections/green-software/index.md

@@ -4,6 +4,7 @@ items:
  - kube-green/kube-green
  - marmelab/greenframe-cli
  - ThijsRay/coppers
+ - github/GreenSoftwareDirectory
 
 display_name: Green Software
 created_by: tomthorogood