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Problem

The changelog generator does not have a way to add external links to it. For example, when a new version of Spring Boot is released, it is accompanied by release notes on the Github Wiki. It would be nice if the changelog could point to these release notes so that they are easy to discover.

Solution

Add an option to add external links at the bottom of the changelog. These links should be configurable using a property as follows:

changelog:
  external_links:
  - name: Release notes
    location: https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-2.3-Release-Notes

The content for the changelog is generated here. The tests for verifying the output can be found here.

## Steps to Fix 
 
- [ ] Claim this issue with a comment below and ask any clarifying questions you need
- [ ] Set up a repository locally following the [Contributing Guidelines](https://github.com/spring-io/github-changelog-generator/blob/master/CONTRIBUTING.adoc)
- [ ] Try to fix the issue following the steps above
- [ ] Commit your changes and start a pull request.