start docs of new plugin features

docs/advancedPlugin.md

@@ -0,0 +1,38 @@
+```json
+    "CustomRequirement": [
+      {
+        rule: "custom",
+        plugin: "My Plugin Name",
+        name: "checkThing",
+        otherStuff: "I like nachos"
+      }
+    ]
+```
+
+plugin
+```js
+const addOptionalRules = require('./helpers/addOptionalRules')
+
+module.exports = (context) => {
+  // Register this plugin
+  context.addPlugin({
+    name: 'My Plugin Name',
+    description: 'I do amazing things',
+    customChecks: {
+      checkThing: async (rule, context) => {
+        const successObject = {
+          pass: true,
+          message: 'Custom check succeeded!'
+        }
+        const failureObject = {
+          pass: false,
+          message: 'Custom check failed!'
+        }
+
+        // Randomly succeed or fail
+        return !!Math.floor(Math.random() * 2) ? successObject : failureObject
+      }
+    }
+  })
+}
+```