Update README.md

Author: ksw2000

README.md

@@ -1,6 +1,6 @@
 # hackmd-to-html-cli
 
-[![NPM version](https://img.shields.io/npm/v/hackmd-to-html-cli.svg?logo=npm&style=flat-square)](https://www.npmjs.org/package/hackmd-to-html-cli) ![](https://img.shields.io/github/license/ksw2000/hackmd-to-html-cli?color=yellow&style=flat-square) ![](https://img.shields.io/github/actions/workflow/status/ksw2000/hackmd-to-html-cli/publish.yml?branch=main&style=flat-square) ![](https://img.shields.io/npm/dt/hackmd-to-html-cli?color=blue&style=flat-square)
+[![NPM version](https://img.shields.io/npm/v/hackmd-to-html-cli.svg?logo=npm&style=flat-square)](https://www.npmjs.org/package/hackmd-to-html-cli) ![](https://img.shields.io/github/license/ksw2000/hackmd-to-html-cli?color=yellow&style=flat-square) ![](https://img.shields.io/github/actions/workflow/status/ksw2000/hackmd-to-html-cli/gitpage.yml?branch=main&style=flat-square) ![](https://img.shields.io/npm/dt/hackmd-to-html-cli?color=blue&style=flat-square)
 
 
 A simple Node.js wrapper for `markdown-it`. This tool helps to convert HackMD markdown files to HTML files.
@@ -100,4 +100,5 @@ $ hmd2html -s hello.md -l ./myLayout.html
     + YAML Metadata
     + Render CSV as table
     + PlantUML
-    + Fretboard
+    + Fretboard
+    + Support externals