Add UPGRADING.md (#51)

* Add UPGRADING.md

* level -> level-browserify

Author: Lars-Magnus Skog

README.md

@@ -19,6 +19,8 @@ Use this package to avoid having to explicitly install `leveldown`/`level-js` wh
 
 In node.js you get `leveldown`, while in the browser you get `level-js` (through use of browserify's `browser` field setting in `package.json`).
 
+**If you are upgrading:** please see [`UPGRADING.md`](UPGRADING.md).
+
 ## Table of Contents
 
 * [Usage](#usage)

UPGRADING.md

@@ -0,0 +1,28 @@
+# Upgrade Guide
+
+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).
+
+## v2
+
+Dropped support for node 4.
+
+The parts that make up `level-browserify` have been refactored to increase modularity. This is an upgrade to `leveldown@^4.0.0`, `level-js@^3.0.0` and `level-packager@~3.1.0`, which in turn upgraded to `levelup@^3.0.0`. The responsibility of encoding keys and values moved from [`levelup`](https://github.com/Level/levelup) to [`encoding-down`](https://github.com/Level/encoding-down), which comes bundled with [`level-packager`](https://github.com/Level/packager).
+
+Also, upgrading `leveldown` and `level-js` means upgrading to `abstract-leveldown@~5.0.0` which in turn contains breaking changes to [`.batch()`](https://github.com/Level/abstract-leveldown/commit/a2621ad70571f6ade9d2be42632ece042e068805). Though this is negated by `levelup`, we decided to release a new major version in the event of dependents reaching down into `db.db`.
+
+Being a convenience package, `level-browserify` glues the parts back together to form a drop-in replacement for the users of `levelup@1`, while staying fully compatible with `level-browserify@1`. One thing we do get for free, is native Promise support.
+
+```js
+const db = level('db')
+await db.put('foo', 'bar')
+console.log(await db.get('foo'))
+```
+
+This does not affect the existing callback API, functionality-wise or performance-wise.
+
+For more information please check the corresponding `CHANGELOG.md` for:
+
+* [`levelup`](https://github.com/Level/levelup/blob/master/CHANGELOG.md)
+* [`leveldown`](https://github.com/Level/leveldown/blob/master/CHANGELOG.md)
+* [`level-js`](https://github.com/Level/level-js/blob/master/CHANGELOG.md)
+* [`level-packager`](https://github.com/Level/packager/blob/master/CHANGELOG.md)