Changelog gen

[nckturner]

e.g.

$ ./hack/changelog.py \
  --token=$GITHUB_TOKEN \
  --section-title="Section Title" \
  --range=origin/master..v0.5.1-alpha.1 \
  --changelog-file=CHANGELOG.txt

and

$ cat CHANGELOG.txt

Section Title
* Fix CRD mapper blocking all others because caches never sync  and revamp backend-mode flag ([#303](https://github.com/kubernetes-sigs/aws-iam-authenticator/pull/303), @wongma7)
* Update aws-sdk-go to version v1.30.0 ([#306](https://github.com/kubernetes-sigs/aws-iam-authenticator/pull/306), @nckturner)
* Bump k8s.io/ dependencies to 1.16.8 ([#305](https://github.com/kubernetes-sigs/aws-iam-authenticator/pull/305), @wongma7)
* chown aws-iam-authenticator to avoid permission denied ([#302](https://github.com/kubernetes-sigs/aws-iam-authenticator/pull/302), @wongma7)
* Indentation and unit test improvements ([#298](https://github.com/kubernetes-sigs/aws-iam-authenticator/pull/298), @bhagwat070919)
* Adding Rate limiting ec2:DescribeInstances API along with Batching for high TPS ([#292](https://github.com/kubernetes-sigs/aws-iam-authenticator/pull/292), @bhagwat070919)

(Also had to update vendor)

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nckturner

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[wongma7]

cool!

[wongma7]

/lgtm

[christopherhein]

Nice and this could be piped into goreleaser as the release notes. https://goreleaser.com/customization/release/#custom-release-notes

[wongma7]

/lgtm