The instructions for uploading / pasting a diff are found in src/applications/differential/controller/DifferentialDiffCreateController.php:
//The best way to create a Differential diff is by using Arcanist, but you can also just paste a diff (for example, from svn diff, git diff or hg diff --git) into this box, or upload a diff file.//
For the FreeBSD project we'd like to modify or append to this text, e.g. adding a link to project-specific information (https://wiki.freebsd.org/CodeReview). Would it make sense to have this text as a configurable item?