[CI-SKIP] Add PR rebasing steps (#5634)

CONTRIBUTING.md

@@ -21,6 +21,7 @@ To generate and update the TOC: https://github.com/mzlogin/vim-markdown-toc -->
     * [Manual method](#manual-method)
     * [Automatic method](#automatic-method)
 * [PR Policy](#pr-policy)
+* [Rebasing PRs](#rebasing-prs)
 * [Formatting](#formatting)
 * [Patch Notes](#patch-notes)
 * [Obfuscation Helpers](#obfuscation-helpers)
@@ -238,6 +239,21 @@ need to "save" the changes.
    - This will modify the appropriate patches based on your commits.
 1. PR your modified patch file(s) back to this repository.
 
+## Rebasing PRs
+
+Steps to rebase a PR to include the latest changes from `master`.  
+These steps assume the `origin` remote is your fork of this repository and `upstream` is the official PaperMC repository.
+
+1. Pull latest changes from upstream's master: `git checkout master && git pull upstream master`.
+1. Checkout feature/fix branch and rebase on master: `git checkout patch-branch && git rebase master`.
+1. Apply updated patches: `./paper patch`.
+1. If there are conflicts, fix them.
+1. If your PR creates new patches instead of modifying exist ones, in both the `Paper-Server` and `Paper-API` directories, ensure your newly-created patch is the last commit by either:
+    * Renaming the patch file with a large 4 digit number in front (e.g. 9999-Patch-to-add-some-new-stuff.patch)
+    * Run `git rebase --interactive upstream/upstream` and move the commits to the end.
+1. Rebuild patches: `./paper rebuild`.
+1. Force push changes: `git push --force`.
+
 ## PR Policy
 
 We'll accept changes that make sense. You should be able to justify their