Add polyfill.io cheatsheet

polyfill.io.md

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+---
+title: Polyfill.io
+category: JavaScript libraries
+updated: 2017-10-26
+layout: 2017/sheet
+intro: |
+  [Polyfill.io](https://polyfill.io) is a service that serves JavaScript polyfills.
+---
+
+## Usage
+
+### Default usage
+
+```html
+<script src="https://cdn.polyfill.io/v2/polyfill.min.js"></script>
+```
+{: .-wrap}
+
+This is the default script for Polyfill.io.
+
+### References
+
+- [API example](https://polyfill.io/v2/docs/api) _(polyfill.io)_
+- [List of features](https://polyfill.io/v2/docs/features) _(polyfill.io)_
+
+## Optimized
+
+### For modern browsers
+
+```html
+<script>(function(d,s){
+if(window.Promise&&[].includes&&Object.assign&&window.Map)return;
+var sc=d.getElementsByTagName(s)[0],js=d.createElement(s);
+js.src='https://cdn.polyfill.io/v2/polyfill.min.js';
+sc.parentNode.insertBefore(js, sc);
+}(document,'script'))</script>
+```
+
+This only includes polyfill.io when necessary, skipping it for modern browsers for faster load times.
+
+### Extra features
+
+```html
+<script>(function(d,s){
+if(window.fetch&&window.Promise&&[].includes&&Object.assign&&window.Map)return;
+var sc=d.getElementsByTagName(s)[0],js=d.createElement(s);
+js.src='https://cdn.polyfill.io/v2/polyfill.min.js?features=default,fetch';
+sc.parentNode.insertBefore(js, sc);
+}(document,'script'))</script>
+```
+{: data-line="2,4"}
+
+This is the same as the previous, but also adds a polyfill for `window.fetch()`. We add a `window.fetch` check and loads the additional `fetch` feature.