How to take someone's PR and make your own based off it while keeping their commit:

1. Make a new branch, preferibly with the PR# of the original one.

git checkout -b pr/xyz

2. Pull the changes to it.

git pull git://github.com/rafase282/wiki.git update/pr-guide

This basically means pull from the repository Rafase282/wiki, branch update/pr/guide into your current branch pr/xyz.

3. Make your changes, add, commit, push. If you need to squash make sure to keep the original contributor's commit untouched.

Note: You might need to force push git push -f origin pr/xyz

4. Create your PR and in the description make it to close the original PR with closes #xyz

That should make a new pull request with the original changes, plus your own into a new pull request that will automatically reference and close the original one when merged.