feat: use conventional config when using zero configuration

@commitlint/cli/README.md

@@ -8,6 +8,14 @@
 
 ## Getting started
 
+Commitlint works with zero configuration, it uses the `@commitlint/config-conventional` by default.
+
+```
+npm install --save-dev @commitlint/cli
+```
+
+If you want to use other conventions, you can to specify the configuration you want to use.
+
 ```
 npm install --save-dev @commitlint/cli @commitlint/config-angular
 echo "module.exports = {extends: ['@commitlint/config-angular']};" > commitlint.config.js

@commitlint/cli/package.json

@@ -72,6 +72,7 @@
     "tmp": "0.0.33"
   },
   "dependencies": {
+    "@commitlint/config-conventional": "^7.5.0",
     "@commitlint/format": "^7.5.0",
     "@commitlint/lint": "^7.5.2",
     "@commitlint/load": "^7.5.0",

@commitlint/cli/src/cli.js

@@ -281,8 +281,11 @@ function getSeed(seed) {
 	const e = Array.isArray(seed.extends) ? seed.extends : [seed.extends];
 	const n = e.filter(i => typeof i === 'string');
 	return n.length > 0
-		? {extends: n, parserPreset: seed.parserPreset}
-		: {parserPreset: seed.parserPreset};
+		? {parserPreset: seed.parserPreset, extends: n}
+		: {
+				parserPreset: seed.parserPreset,
+				extends: ['@commitlint/config-conventional']
+		  };
 }
 
 function selectParserOpts(parserPreset) {

@commitlint/cli/src/cli.test.js

@@ -46,10 +46,10 @@ test('should produce no success output with -q flag', async t => {
 	t.is(actual.stderr, '');
 });
 
-test('should fail for input from stdin without rules', async t => {
+test('should succeed for input from stdin without configuration', async t => {
 	const cwd = await git.bootstrap('fixtures/empty');
-	const actual = await cli([], {cwd})('foo: bar');
-	t.is(actual.code, 1);
+	const actual = await cli([], {cwd})('feat: support zero configuration');
+	t.is(actual.code, 0);
 });
 
 test('should succeed for input from stdin with rules', async t => {

README.md

@@ -77,13 +77,12 @@ These can be modified by [your own configuration](#config).
 ## Getting started
 
 ```sh
-# Install commitlint cli and conventional config
-npm install --save-dev @commitlint/{config-conventional,cli}
-# For Windows:
-npm install --save-dev @commitlint/config-conventional @commitlint/cli
+# Install commitlint cli
+npm install --save-dev @commitlint/cli
 
-# Configure commitlint to use conventional config
-echo "module.exports = {extends: ['@commitlint/config-conventional']}" > commitlint.config.js
+# Configure commitlint to use other conventions
+npm install --save-dev @commitlint/cli @commitlint/config-angular
+echo "module.exports = {extends: ['@commitlint/config-angular']}" > commitlint.config.js
 ```