Document strict mode in readme

Author: lukechilds

README.md

@@ -7,6 +7,43 @@ This module is used for writing unit tests for your applications, you can access
 It aims to be fully compatibe with the [node.js assert module](http://nodejs.org/api/assert.html), same API and same behavior, just adding support for web browsers.
 The API and code may contain traces of the [CommonJS Unit Testing 1.0 spec](http://wiki.commonjs.org/wiki/Unit_Testing/1.0) which they were based on, but both have evolved significantly since then.
 
+A `strict` and a `legacy` mode exist, while it is recommended to only use `strict mode`.
+
+## Strict mode
+
+When using the `strict mode`, any `assert` function will use the equality used in the strict function mode. So `assert.deepEqual()` will, for example, work the same as `assert.deepStrictEqual()`.
+
+It can be accessed using:
+
+```js
+const assert = require('assert').strict;
+```
+
+## Legacy mode
+
+> Deprecated: Use strict mode instead.
+
+When accessing `assert` directly instead of using the `strict` property, the
+[Abstract Equality Comparison](https://tc39.github.io/ecma262/#sec-abstract-equality-comparison) will be used for any function without a
+"strict" in its name (e.g. `assert.deepEqual()`).
+
+It can be accessed using:
+
+```js
+const assert = require('assert');
+```
+
+It is recommended to use the `strict mode` instead as the Abstract Equality Comparison can often have surprising results. Especially
+in case of `assert.deepEqual()` as the used comparison rules there are very lax.
+
+E.g.
+
+```js
+// WARNING: This does not throw an AssertionError!
+assert.deepEqual(/a/gi, new Date());
+```
+
+
 ## assert.fail(actual, expected, message, operator)
 Throws an exception that displays the values for actual and expected separated by the provided operator.