Add decription for data prop

types/vfile/readme.md

@@ -3,6 +3,34 @@
 Below are some examples of how to use these Definitions.<br>
 Scenarios where the Typescript code is identical to plain Javascript code are omitted.
 
+### Using `data` property
+
+Because `data` property type is set `unknown`, you have to make your own type guard.
+
+```ts
+interface CustomData {
+    message: string;
+}
+// This is the type guard
+function isCustomData(data: any): data is CustomData {
+    return data != null && typeof data.message === 'string';
+}
+function doSomethingWithCustomData(data: CustomData) {
+}
+const customData: CustomData = {
+    message: 'tango'
+};
+const fileWithData = vfile({
+    data: customData
+});
+
+if (isCustomData(fileWithData.data)) {
+    doSomethingWithCustomData(fileWithData.data);
+}
+// The below code will throw a compiler error
+doSomethingWithCustomData(fileWithData.data); // $ExpectError
+```
+
 ### Extending VFile
 
 ```ts