If you're looking to sign the open letter via a pull request, you should visit this link. The file you want to edit is the README.md.
If you're looking to add something to the website itself, continue reading below.
The website is built using:
Fork, then clone the repo:
git clone git@github.com:your-username/alp-fail.git
Install all the dependencies:
npm install
Then, run this command so you can make changes and see them when saving:
gatsby develop
After making your changes, push to your fork and submit a pull request.