GNOME CEC Link

[kwurst]

Make the phrase "GNOME Community Engagement Challenge" into a link in the Acknowledgements at the bottom of the README.md file.

The URL to use for the link is: https://www.gnome.org/challenge/

The "Links" section of the following document shows how to create a link using Markdown:

• https://docs.github.com/en/github/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax