Added playground to the PR checklist

.github/pull_request_template.md

@@ -7,7 +7,7 @@ Provide a detailed summary of your PR. Explain how you arrived at your solution.
 - [ ] Check against **all themes** for compatibility in both light and dark modes
 - [ ] Checked in **mobile**
 - [ ] Checked in **Chrome**, **Safari**, **Edge**, and **Firefox**
-- [ ] Props have proper **autodocs**
+- [ ] Props have proper **autodocs** and **playground** toggles
 - [ ] Added **[documentation](https://github.com/elastic/eui/blob/master/wiki/documentation-guidelines.md)**
 - [ ] Checked **[Code Sandbox](https://codesandbox.io/)** works for the any docs examples
 - [ ] Added or updated **[jest tests](https://github.com/elastic/eui/blob/master/wiki/testing.md)**

wiki/documentation-guidelines.md

@@ -128,6 +128,46 @@ There are a couple themes to keep in mind when adding snippets:
 />
 ```
 
+## Adding playground toggles
+
+Most documentation pages include a [playground section](https://elastic.github.io/eui/#/layout/accordion/playground) where consumers can interact with the component's props to see in real time how different configurations affect visual and functional output. Generally, the playground system will automatically generate the correct toggle type; for instance, a text input for props that accept string values, and a switch input for props that accept boolean values.
+
+### Toggles for required props
+
+Props marked required for a component typically do not have default values and therefore need to be set for the playground to work well. For example, the `children` prop, which can be set in the component's [`playground.js` file](https://github.com/elastic/eui/blob/master/src-docs/src/views/accordion/playground.js):
+
+```js
+propsToUse.children = {
+  value: `<EuiText>
+    <p>
+      Any content inside of <strong>EuiAccordion</strong> will appear here.
+    </p>
+  </EuiText>`,
+  type: PropTypes.ReactNode,
+  hidden: false,
+};
+```
+### Custom or altered toggles
+
+Some props accept values that are difficult to parse or require knowledge about how the prop should be used to determine the type of toggle to use. For example, callback function props such as `onToggle`. For cases like this we may provide utility functions to help:
+
+```js
+propsToUse.onToggle = simulateFunction(propsToUse.onToggle);
+```
+
+Or perhaps the prop accepts a wide range of values and the best user experience would be to limit the value to a simpler input:
+
+```js
+propsToUse.valueAppend = {
+  ...propsToUse.valueAppend,
+  type: PropTypes.String,
+};
+```
+
+### Toggles for complex or markup-heavy props
+
+Not all props lend themselves to becoming helpful playground toggles. For instance, optional "action" props that require the consumer to provide a fully configured button or link element. In cases such as this, it is acceptable to omit the toggle and rely on prop table documentation to convey how the prop is best used.
+
 ## Changelog
 
 Any updates to the `src/` folder require an entry in the [CHANGELOG.md](../CHANGELOG.md) file. Documentation-only changes do not. Here are our guidelines for updating the file: