Include instruction how to extend vfiles

types/vfile/readme.md

@@ -0,0 +1,32 @@
+## VFile typescript Docs
+
+Below are some examples of how to use these Definitions.<br>
+Scenarios where the Typescript code is identical to plain Javascript code are omitted.
+
+### Extending VFile
+
+```ts
+import vfile = require('vfile')
+
+// 1. Define extended vfile interface
+interface CustomVFile extends vfile.VFile {
+    custom: string;
+    data: {
+        custom: number;
+    };
+}
+
+// 2. Provide the extended interface to generic type parameter
+// So it can be used for return value
+const file: CustomVFile = vfile<CustomVFile>({
+    path: '~/example.txt',
+    contents: 'Alpha *braavo* charlie.',
+    custom: 'Custom tango',
+    data: {
+        custom: 12345
+    }
+});
+
+// vfile recognize extended VFile values. So it will return same type.
+const copiedFile: CustomVFile = vfile(file);
+```