Introduce Changelog and PR template (#765)

.github/PULL_REQUEST_TEMPLATE.md

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+Thank you for your contribution!
+
+## Checklist
+- [ ] Feature branch is up-to-date with `master` (if not - rebase it).
+- [ ] Added tests.
+- [ ] Added an entry to the [Changelog](../blob/master/CHANGELOG.md) if the new 
+  code introduces user-observable changes.

CHANGELOG.md

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+# Changelog
+
+## Unreleased
+
+### Breaking Changes
+
+- None
+
+### Added
+
+- None
+
+### Fixed
+
+- None
+
+## 3.0.0 - 2020-06-10
+
+### Breaking Changes
+
+- Cucumber 3 support removed (#762)
+
+### Added
+
+- Cucumber 4 support (#762)
+
+### Fixed
+
+- Fix a bundler deprecation when running specs (#761)
+- remove name override logic that never worked (#758)
+
+### Dependencies
+
+- Drop ruby 2.3 support (#760)
+- Drop ruby 2.2 support (#759)
+
+## Previous versions
+
+No docs yet. Contributions welcome!