Add "drush generate readme" command

[JPustkuchen]

From: drush-ops/drush#5554

I really enjoy using the drush generate command for all sorts of things, as it saves a lot of time, when creating a module. And I recently wondered why there is no drush generate readme command, which would generate a README.md template, using Drupal's README.md template.

Describe the solution you'd like
Implement a drush generate readme command to easily generate a module's README.md.

[Chi-teck]

Added a generator for README.md to 3.x branch.
https://github.com/Chi-teck/drupal-code-generator/tree/3.x/templates/_readme

Thank you.

[JPustkuchen]

Thank you very very much @Chi-teck! Super cool :)

[gitressa]

This is awesome, thanks!