(pkg): add a Changelog that points to the GH Releases page

- Some might not be aware that that's where it's hosted, so be explicit
- Also be explicit that we use SemVer

- in the future things might change and Changelog might be hosted here
  so that it's independent from GitHub
- and Changelog style might change to point out PRs, issues, etc,
  instead of being mostly a list
  - GH Release descriptions are unversioned, so style changes are
    currently fluid as a result

CHANGELOG.md

@@ -0,0 +1,7 @@
+# Changelog
+
+The changelog is currently hosted on [the GitHub Releases page](https://github.com/agilgur5/react-native-manga-reader-app/releases).<br>
+It is currently mostly a summary and list of commits made before any tag.
+The commits in this library mostly follow a convention and tend to be quite detailed.
+
+This project adheres to [Semantic Versioning](http://semver.org/).