add CHANGELOG.md (#166)

This is the initial creation of a CHANGELOG. When we are ready to cut
a release we can change the version at the top and update the links at
the bottom.

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
+
+- For macOS and Windows, build a special version of the binary that can install
+  and manage its own service configuration. You can use this with `nebula
+  -service`.  If you are building from source, use `make service` to build this feature.
+- Support for `mips`, `mips64`, `386` and `ppc64le` processors on Linux.
+- You can now configure the DNS listen host and port with `lighthouse.dns.host`
+  and `lighthouse.dns.port`.
+- Subnet and routing support. You can now add a `unsafe_routes` section to your
+  config to allow hosts to act as gateways to other subnets. Read the example
+  config for more details. This is supported on Linux and macOS.
+
+### Changed
+
+- Certificates now have more verifications performed, including making sure
+  the certificate lifespan does not exceed the lifespan of the root CA. This
+  could cause issues if you have signed certificates with expirations beyond
+  the expiration of your CA, and you will need to reissue your certificates.
+- If lighthouse interval is set to `0`, never update the lighthouse (mobile
+  optimization).
+- Various documentation and example fixes.
+- Improved error messages.
+- Dependencies updated.
+
+### Fixed
+
+- If you have a firewall rule with `group: ["one-group"]`, this will
+  now be accepted, with a warning to use `group: "one-group"` instead.
+- The `listen.host` configuration option was previously ignored (the bind host
+  was always 0.0.0.0). This option will now be honored.
+- The `ca_sha` and `ca_name` firewall rule options should now work correctly.
+
+## [1.0.0] - 2019-11-19
+
+### Added
+
+- Initial public release.
+
+[Unreleased]: https://github.com/slackhq/nebula/compare/v1.0.0...HEAD
+[1.0.0]: https://github.com/slackhq/nebula/releases/tag/v1.0.0