Add example in documentation to run action only for changed files

[ToshY]

Fixes #12

---

1. In order to run the action on just the changed files, we use tj-actions/changed-files to get the diff.
2. Next we construct a list of (intersection) arguments from the changed files.
   1. This code is derived from the PHP-CS-Fixer documentation, section "Using PHP CS Fixer on CI".
   2. As the $EXTRA_ARGS is a multiline string, we add it to $GITHUB_ENV (If there's a better way for this, please let me know).
3. We pass the arguments to PHP-CS-Fixer using ${{ env.PHPCS_EXTRA_ARGS }}".

---

As sidenote, if you do not want or use this action but instead have php-cs-fixer as dev dependency in your application, the steps are somewhat simpler as you can do number 2 and 3 in a single step without the need for saving the variable to the environment.

      - name: Get changed files
        id: changed-files
        uses: tj-actions/changed-files@v38

      - name: PHP CS Fixer
        run: |
          CHANGED_FILES=$(echo "${{ steps.changed-files.outputs.all_changed_and_modified_files }}" | tr ' ' '\n')
          if ! echo "${CHANGED_FILES}" | grep -qE "^(\\.php-cs-fixer(\\.dist)?\\.php|composer\\.lock)$"; then EXTRA_ARGS=$(printf -- '--path-mode=intersection\n--\n%s' "${CHANGED_FILES}"); else EXTRA_ARGS=''; fi
          vendor/bin/php-cs-fixer fix --config=.php-cs-fixer.dist.php -v --dry-run --diff --using-cache=no ${EXTRA_ARGS}

[OskarStark]

Thank you 🙏