Add a changelog, require its maintenance

Problem: we are going to release this tool and from that point
it would be nice to maintain its changelog.

Solution: add a simple CHANGES.md and update PR template.

.github/pull_request_template.md

@@ -38,6 +38,8 @@ checkmark indicating that you are sure it is dealt with (be that by irrelevance)
 - Public contracts
   - [ ] Any modifications of public contracts comply with the [Evolution
   of Public Contracts](https://www.notion.so/serokell/Evolution-of-Public-Contracts-2a3bf7971abe4806a24f63c84e7076c5) policy.
+  - [ ] I added an entry to the [changelog](/CHANGES.md) if my changes are visible to the users
+        and
   - [ ] provided a migration guide for breaking changes if possible
 
 #### Stylistic guide (mandatory)

CHANGES.md

@@ -0,0 +1,13 @@
+<!--
+ - SPDX-FileCopyrightText: 2020 Serokell <https://serokell.io>
+ -
+ - SPDX-License-Identifier: MPL-2.0
+ -->
+
+Unreleased
+==========
+
+0.1.0.0
+=======
+
+Initial release.

package.yaml

@@ -13,6 +13,7 @@ copyright:           2018-2019 Serokell <https://serokell.io>
 
 extra-source-files:
 - README.md
+- CHANGES.md
 - src-files/*
 
 description:         Please see the README on GitHub at <https://github.com/serokell/xrefcheck#readme>