Add notice about browser support.

README.md

@@ -10,6 +10,10 @@ With browserify, simply `require('assert')` or use the `assert` global and you w
 
 The goal is to provide an API that is as functionally identical to the [Node.js `assert` API](https://nodejs.org/api/assert.html) as possible. Read the [official docs](https://nodejs.org/api/assert.html) for API documentation.
 
+> `assert` supports modern browsers. Older browsers may require shimming functionality.
+> In Internet Explorer and other environments without `Promise`, the `throws`/`rejects` family of methods requires a Promise shim.
+> In Internet Explorer 8-10, the shim and sham versions of [es5-shim](https://github.com/es-shims/es5-shim) are also required.
+
 ## Install
 
 To use this module directly (without browserify), install it as a dependency: