Prepare changelog and upgrade guide for v4.0.0 (#628)

CHANGELOG.md

@@ -2,6 +2,20 @@
 
 ## [Unreleased]
 
+## [4.0.0] - 2018-12-22
+
+### Changed
+* Upgrade `nyc` devDependency from `~12.0.2` to `~13.1.0` (@ralphtheninja)
+* Upgrade `deferred-leveldown` dependency from `~4.0.0` to `~5.0.0` (@vweevers)
+* Upgrade `concat-stream` devDependency from `~1.6.0` to `~2.0.0` (@ralphtheninja)
+* Upgrade `level-iterator-stream` dependency from `~3.0.0` to `~4.0.0` (@ralphtheninja)
+* Replace `remark-cli` with `hallmark` (#621) (@vweevers)
+* Upgrade `standard` devDependency from `^11.0.0` to `^12.0.0` (@ralphtheninja)
+* Add `.nyc_output/` to `.npmignore` (@ralphtheninja)
+
+### Removed
+* Remove `IE10` from `airtap` (#625) (@ralphtheninja)
+
 ## [3.1.1] - 2018-07-14
 
 ### Changed
@@ -995,7 +1009,9 @@
 
 :seedling: Initial release.
 
-[unreleased]: https://github.com/level/levelup/compare/v3.1.1...HEAD
+[unreleased]: https://github.com/level/levelup/compare/v4.0.0...HEAD
+
+[4.0.0]: https://github.com/level/levelup/compare/v3.1.1...v4.0.0
 
 [3.1.1]: https://github.com/level/levelup/compare/v3.1.0...v3.1.1
 

UPGRADING.md

@@ -2,6 +2,12 @@
 
 This document describes breaking changes and how to upgrade. For a complete list of changes including minor and patch releases, please refer to the [changelog](CHANGELOG.md).
 
+## v4
+
+There have been two major updates to dependencies. First, `level-iterator-stream` is now based on [`readable-stream@3`](https://github.com/nodejs/readable-stream#version-3xx). Second, `deferred-leveldown` is now based on [`abstract-leveldown@6`](https://github.com/Level/abstract-leveldown/blob/master/UPGRADING.md#v6). Please follow these links for more information; both contain significant enough changes to warrant this `levelup` major. In addition, all aforementioned dependencies and by extension `levelup` have dropped support of IE10. 
+
+To get a consistent behavior between opening and opened `levelup` instances (in the former case, your store will be wrapped with `deferred-leveldown`), we recommend to pair `levelup@4` only with a store based on `abstract-leveldown` >= 6. For example, `deferred-leveldown` now rejects `null` and `undefined` values. If you pair `levelup@4` with an older store, `db.put('key', null)` would only throw an error if `db` is still opening itself.
+
 ## v3
 
 1. Dropped support for node 4.