add Makefile to generate changelog

Makefile

@@ -0,0 +1,18 @@
+tag-changelog: require-version require-gh-token
+	echo "Tagging..." && \
+	git tag -a "$$VERSION" -f --annotate -m"Tagged $$VERSION" && \
+	git push --tags -f && \
+	git checkout master && \
+	git pull && \
+	github_changelog_generator --no-issues --max-issues 100 --token "${GH_TOKEN}" --user getlantern --project systray && \
+	git add CHANGELOG.md && \
+	git commit -m "Updated changelog for $$VERSION" && \
+	git push origin HEAD && \
+	git checkout -
+
+guard-%:
+	 @ if [ -z '${${*}}' ]; then echo 'Environment variable $* not set' && exit 1; fi
+
+require-version: guard-VERSION
+
+require-gh-token: guard-GH_TOKEN

README.md

@@ -28,6 +28,8 @@ func onExit() {
 }
 ```
 
+See [full API](https://pkg.go.dev/github.com/getlantern/systray?tab=doc) as well as [CHANGELOG](https://github.com/getlantern/systray/tree/master/CHANGELOG.md).
+
 ## Try the example app!
 
 Have go v1.12+ or higher installed? Here's an example to get started on macOS: