Updating CHANGELOG.md format

I started exploring [github_changelog_generator][1] after learning of
it from =bess. This update pairs with a future commit in which I plan
to write the release procedures, which uses
[github_changelog_generator][1].

~enhancement

[1]:https://rubygems.org/gems/github_changelog_generator

CHANGELOG.md

@@ -1,4 +1,10 @@
-v0.1.0 / 2020-02-26
-===================
+# Changelog
 
-  * Initial commit
+## [v0.1.0](https://github.com/samvera-labs/huborg/tree/v0.1.0) (2020-02-26)
+
+[Full Changelog](https://github.com/samvera-labs/huborg/commit/cc1ce07147ad552cabf343c8195610bbb8c9376d)
+
+- Adding Huborg::Client\#push\_template! ([jeremyf](https://github.com/jeremyf))
+
+
+\* *This Changelog was automatically generated by [github_changelog_generator](https://github.com/github-changelog-generator/github-changelog-generator)*