Specifiy release event to publish to PyPi (#30) (#41)

This patch adds a trigger event to publish.yml and specifies the release
event to when we will publish to PyPi.
diff --git a/.github/workflows/publish.yml b/.github/workflows/publish.yml
index 90b2057..693fd17 100644
--- a/.github/workflows/publish.yml
+++ b/.github/workflows/publish.yml
@@ -1,3 +1,7 @@
+on:
+  release:
+    types: [released]
+
 jobs:
   pypi-publish:
     name: Upload release to PyPI