Add CHANGELOG template
### Description
Include a template of a changelog when clicking on `Add Changelog` button on a repository.
### Proposal
When a user clicks on `Add Changelog` button he is redirected to an add file with a pre-loaded template, for example from [here](http://keepachangelog.com/en/0.3.0/).
### Links / references
http://keepachangelog.com/en/0.3.0/
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