fix readme, avoid issues with composer installs

``` 
composer require --prefer-source --no-interaction dompdf/dompdf:dev-master
Warning: This development build of composer is over 30 days old. It is recommended to update it by running "/usr/local/bin/composer self-update" to get the latest version.
composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
  - Installing phenx/php-font-lib (0.2.1)
    Cloning 42a1ca6d19f14076911a118705b771c3a5a8b179



  [RuntimeException]
  Failed to execute git checkout '42a1ca6d19f14076911a118705b771c3a5a8b179' && git reset --hard '42a1ca6d19f14076911a118705b771c3a5a8b179'

  error: Your local changes to the following files would be overwritten by checkout:
        README.md
  Please, commit your changes or stash them before you can switch branches.
  Aborting




require [--dev] [--prefer-source] [--prefer-dist] [--no-progress] [--no-update] [packages1] ... [packagesN]
```