Enhance README.md installation documentation

Rather than manually editing your composer.json file, it is also possible to use the composer cli utility itself to manage the composer.json's format, as follows:

```
composer config repositories.drush_cmi_tools \
  '{ "type": "git", "url": "https://github.com/previousnext/drush_cmi_tools.git" }'
```
before running the composer require statement.  
