Add README

Author: thymikee

README.md

@@ -0,0 +1,82 @@
+# snapshot-diff
+Diffing snapshot utility for Jest. Takes two values, and return their difference as a string, ready to be snapshoted with `toMatchSnapshot()`.
+Especially helpful when testing the difference between different states of a React component.
+
+## Installation
+```bash
+yarn add --dev snapshot-diff
+```
+
+## Usage
+
+```js
+const snapshotDiff = require('snapshot-diff');
+
+test('snapshot difference between 2 strings', () => {
+  expect(snapshotDiff(a, b).toMatchSnapshot();
+});
+
+const React = require('react');
+const Component = require('./Component');
+
+test('snapshot difference between 2 React components state', () => {
+  expect(
+    snapshotDiff(
+      <Component test="say" />,
+      <Component test="my name" />
+    )
+  ).toMatchSnapshot();
+});
+```
+
+Produced snapshot:
+```diff
+exports[`diffs short strings 1`] = `
+"- First value
++ Second value
+
+
+-  abcx
++  abcy
+   "
+`;
+
+exports[`detects React components 1`] = `
+"- <Component test=\\"say\\" />
++ <Component test=\\"my name\\" />
+
+@@ -27,11 +27,11 @@
+   <span />
+   <span />
+   <span />
+   <span />
+   <span>
+-    say
++    my name
+   </span>
+   <span />
+   <span />
+   <span />
+   <span />"
+`;
+```
+
+## API
+
+```js
+type Options = {
+  expand?: boolean,
+  colors?: boolean
+};
+
+// default export
+snapshotDiff(valueA: any, valueB: any, options?: Options) => string
+```
+
+### Options
+* `expand: boolean` (default: `false`) – expand the diff, so the whole information is preserved
+* `colors: boolean` (default: `false`) – preserve color information from Jest diff
+
+---
+
+Project is MIT-licensed. Pull Requests welcome!