Document how to generate release notes

[vincepri]

In the https://github.com/kubernetes-sigs/cluster-api/blob/master/docs/developer/releasing.md document, we need a section on how to generate release notes.

/kind documentation
/area release
/milestone v0.3.0
/help

[k8s-ci-robot]

@vincepri:
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In response to this:

In the https://github.com/kubernetes-sigs/cluster-api/blob/master/docs/developer/releasing.md document, we need a section on how to generate release notes.

/kind documentation
/area release
/milestone v0.3.0
/help

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[noamran]

/assign