Improve changelog (#35)

* New, improved, standardized CHANGELOG

* Mentioned new changelog in contribution guidelines

* Formatting, working links, new version number

Co-authored-by: Rich Vanderwal <rvanderwal@newrelic.com>

CHANGELOG.md

@@ -1,5 +1,26 @@
-## ChangeLog
+# Changelog
+All notable changes to this project will be documented in this file.
 
-## 0.1.0
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
+## [Unreleased]
+
+## [0.13.0] - 2020-10-28
+### Added
+- Support for metrics (#10)
+- Version number has been modified to track the version numbers of the
+  go.opentelemetry.io/otel upstream library.
+
+### Changed
+- Updated to use version 0.13.0 of the go.opentelemetry.io/otel packages. (#30)
+- Standardized CHANGELOG.md format. When making changes to this project, add
+  human-readable summaries of what you've done to the "Unreleased" section
+  above. When creating a release, move that information into a new release
+  section in this document. (#35)
+
+## [0.1.0] - 2019-12-31
 First release!
+
+[Unreleased]: https://github.com/newrelic/opentelemetry-exporter-go/compare/v0.13.0...HEAD
+[0.13.0]: https://github.com/newrelic/opentelemetry-exporter-go/compare/v0.1.0...v0.13.0
+[0.1.0]: https://github.com/newrelic/opentelemetry-exporter-go/releases/tag/v0.1.0

CONTRIBUTING.md

@@ -37,6 +37,8 @@ This Open Source Software can be used in a large number of environments, all of
 ## Pull Request Guidelines
 Before we can accept a pull request, you must sign our [Contributor Licensing Agreement](#contributor-license-agreement-cla), if you have not already done so. This grants us the right to use your code under the same Apache 2.0 license as we use for this project in general.
 
+If this is a notable change, please include a very short summary of your work in the "Unreleased" section of [CHANGELOG.md](./CHANGELOG.MD).
+
 ## Coding Style Guidelines
 Our code base is formatted according to [gofmt](https://golang.org/cmd/gofmt/) and linted with [golint](https://github.com/golang/lint).