Add CHANGELOG.md, documenting important changes

CHANGELOG.md

+# Changelog
+All notable changes to this project will be documented in this file.
+
+## Unreleased
+### Added
+- Support for leases, which can be used as locks with timeouts, allowing QuarkDB to serve as a distributed lock manager.
+- Commands `lease-acquire`, `lease-get`, `lease-release`.
+
+### Changed
+- A newly elected leader now stalls writers in addition to readers, until its leadership marker entry in the raft journal has been committed and applied.
+
+### Fixed
+- A particularly rare race condition was able to trigger an assertion in the Raft subsystem, causing the current cluster leader to crash.
+
+## 0.2.7 (2018-06-22)
+### Added
+- Updated rocksdb dependency to 5.13.4.
+
+### Fixed
+- Certain unlikely sequences of pipelined writes were able to trigger an assertion and bring a cluster down, when part of a transaction. Without that assertion, the commands would have left ghost key-value pairs in the rocksdb keyspace.