sp-dev-fx-webparts/samples/js-extend-webpack/gulpfile.js

'use strict';

const gulp = require('gulp');
const build = require('@microsoft/sp-build-web');

build.configureWebpack.mergeConfig({ 
  additionalConfiguration: (generatedConfiguration) => { 
    generatedConfiguration.module.loaders.push([ 
      { test: /\.md$/, loader: "html!markdown" } 
    ]); 

    return generatedConfiguration; 
  } 
});

build.initialize(gulp);