message: prepare changelog for 3.0.1
diff: ## [Unreleased]
  +## [3.0.1] - 2018-05-22
  +
  +### Added
  +* Add upgrade guide (@ralphtheninja)
  +
  +### Changed
  +* Upgrade to `abstract-leveldown@~5.0.0` (@ralphtheninja)
  +
  ## [3.0.0] - 2018-05-21
  ### Added
  **Historical Note** Earlier versions were published before `v1.0.0` but the code was then a branch inside [`leveldown`](https://github.com/level/leveldown). This version marks the point where that code was extracted into its own repository thanks to the work of [`@mcollina`](https://github.com/mcollina).
  -[Unreleased]: https://github.com/level/rocksdb/compare/v3.0.0...HEAD
  +[Unreleased]: https://github.com/level/rocksdb/compare/v3.0.1...HEAD
  +[3.0.1]: https://github.com/level/rocksdb/compare/v3.0.0...v3.0.1
  [3.0.0]: https://github.com/level/rocksdb/compare/v2.0.0...v3.0.0
  [2.0.0]: https://github.com/level/rocksdb/compare/v1.1.0...v2.0.0
  [1.1.0]: https://github.com/level/rocksdb/compare/v1.0.1...v1.1.0
  