Fixes mozilla-mobile#643 - Adds a changelog and pull reuqest template

.github/PULL_REQUEST_TEMPLATE

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+
+
+### Pull Request checklist
+<!-- Before submitting the PR, please address each item -->
+- [ ] **Quality**: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
+- [ ] **Tests**: This PR includes thorough tests or an explanation of why it does not
+- [ ] **Changelog**: This PR includes [a changelog entry](https://github.com/mozilla-mobile/fenix/blob/master/CHANGELOG.md) or does not need one
+- [ ] **Accessibility**: The code in this PR follows [accessibility best practices](https://github.com/mozilla-mobile/shared-docs/blob/master/android/accessibility_guide.md) or does not include any user facing features

CHANGELOG.md

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+# Changelog
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+### Added
+### Changed
+### Removed