There are no some common guidelines yet. But a lot of good rules are explained on keep a changelog.
And as they mention:
Don’t let your friends dump git logs into changelogs.
And I fully agree with it.
That's why I created GitHub Changelog Generator that generates a changelog according all these guidelines.
Actually it works with GitHub only, but it present a good example of how a changelog should look like.
It automatically generates a changelog from your tags, issues, labels and pull requests.
For example: This changelog was generated by this script: CHANGELOG.md
Changelog
1.2.5 (2015-01-15)
Implemented enhancements:
- Use milestone to specify in which version bug was fixed #22
Fixed bugs:
- Error when trying to generate log for repo without tags #32
Merged pull requests:
PrettyPrint class is included using lowercase 'pp' #43 (schwing)
support enterprise github via command line options #42 (glenlovett)