Remove license note in README

For pull requests, it is very much appreciated to check your Puppet manifest

with [puppet-lint](https://github.com/camptocamp/puppet-postfix/issues) to follow the recommended Puppet style guidelines from the

[Puppet Labs style guide](http://docs.puppetlabs.com/guides/style_guide.html).

## License

Copyright (c) 2015 <mailto:puppet@camptocamp.com> All rights reserved.

    This program is free software: you can redistribute it and/or modify

    it under the terms of the GNU General Public License as published by

    the Free Software Foundation, either version 3 of the License, or

    (at your option) any later version.

    This program is distributed in the hope that it will be useful,

    but WITHOUT ANY WARRANTY; without even the implied warranty of

    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License

    along with this program.  If not, see <http://www.gnu.org/licenses/>.