chore: add changelog template (#9)

templates/CHANGELOG.md.j2

@@ -0,0 +1,19 @@
+# Changelog
+
+{%- for version, release in context.history.released.items() %}
+
+## {{ version.as_tag() }} ({{ release.tagged_date.strftime("%Y-%m-%d") }})
+
+{%- for category, commits in release["elements"].items() %}
+{# Category title: Breaking, Fix, Documentation #}
+### {{ category | capitalize }}
+{# List actual changes in the category #}
+{%- for commit in commits %}
+{% if commit is not none and commit.descriptions is defined %}
+- {{ commit.descriptions[0] | capitalize }} ([`{{ commit.short_hash }}`]({{ commit.hexsha | commit_hash_url }}))
+{% endif %}
+{%- endfor %}{# for commit #}
+
+{%- endfor %}{# for category, commits #}
+
+{%- endfor %}{# for version, release #}