fix(creator): do not override the README.md generated by plugins (vuejs#5472)

closes vuejs#4802
closes vuejs#4942

Author: haoqunjiang

packages/@vue/cli/__tests__/mock-preset-with-readme/generator.js

@@ -0,0 +1,5 @@
+module.exports = (api, options) => {
+  api.render(files => {
+    files['README.md'] = 'hello'
+  })
+}

packages/@vue/cli/__tests__/mock-preset-with-readme/preset.json

@@ -0,0 +1,5 @@
+{
+  "plugins": {
+    "@vue/cli-plugin-babel": {}
+  }
+}

packages/@vue/cli/__tests__/preset.spec.js

@@ -85,3 +85,23 @@ test('should recognize generator/index.js in a local preset directory by async g
   expect(pkg.scripts).toHaveProperty('testasync')
 })
 
+test('should not override the README.md generated by plugins', async () => {
+  const cwd = path.resolve(__dirname, '../../../test')
+  const name = 'test-preset-readme'
+
+  await create(
+    name,
+    {
+      force: true,
+      git: false,
+      cwd,
+      preset: path.resolve(__dirname, './mock-preset-with-readme')
+    }
+  )
+
+  const readme = await fs.readFile(path.resolve(cwd, name, 'README.md'), 'utf-8')
+  expect(readme).toBe('hello')
+
+  const pkg = require(path.resolve(cwd, name, 'package.json'))
+  expect(pkg.devDependencies).toHaveProperty('@vue/cli-plugin-babel')
+})

packages/@vue/cli/lib/Creator.js

@@ -214,13 +214,15 @@ module.exports = class Creator extends EventEmitter {
       await cb()
     }
 
-    // generate README.md
-    stopSpinner()
-    log()
-    logWithSpinner('📄', 'Generating README.md...')
-    await writeFileTree(context, {
-      'README.md': generateReadme(generator.pkg, packageManager)
-    })
+    if (!generator.files['README.md']) {
+      // generate README.md
+      stopSpinner()
+      log()
+      logWithSpinner('📄', 'Generating README.md...')
+      await writeFileTree(context, {
+        'README.md': generateReadme(generator.pkg, packageManager)
+      })
+    }
 
     // generate a .npmrc file for pnpm, to persist the `shamefully-flatten` flag
     if (packageManager === 'pnpm') {