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.rules.push(
      {
        test: /\.md$/,
        use: [
          {
            loader: 'html-loader'
          },
          {
            loader: 'markdown-loader'
          }
        ]
      }
    );

    return generatedConfiguration;
  }
});

build.initialize(gulp);