feat: Auto-detect readme filename

BREAKING CHANGE: the --readme-file option now has a smart default value

__tests__/__snapshots__/bin-readme.js.snap

@@ -1,5 +1,39 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`readme autodetection of different filenames updates readme.markdown 1`] = `
+"# A title
+
+# API
+
+<!-- Generated by documentation.js. Update this documentation by updating the source code. -->
+
+### Table of Contents
+
+-   [foo](#foo)
+-   [bar](#bar)
+
+## foo
+
+A function with documentation.
+
+**Parameters**
+
+-   \`a\`  {string} blah
+
+Returns **[number](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Number)** answer
+
+## bar
+
+A second function with docs
+
+**Parameters**
+
+-   \`b\`  
+
+# Another section
+"
+`;
+
 exports[`readme command --readme-file 1`] = `
 "# A title
 

__tests__/bin-readme.js

@@ -22,6 +22,29 @@ function documentation(args, options, parseJSON) {
   });
 }
 
+describe('readme autodetection of different filenames', function() {
+  const fixtures = path.join(__dirname, 'fixture/readme');
+  const sourceFile = path.join(fixtures, 'index.js');
+  let d;
+  let removeCallback;
+
+  beforeEach(() => {
+    const dirEntry = tmp.dirSync({ unsafeCleanup: true });
+    d = dirEntry.name;
+    fs.copySync(
+      path.join(fixtures, 'README.input.md'),
+      path.join(d, 'readme.markdown')
+    );
+    fs.copySync(path.join(fixtures, 'index.js'), path.join(d, 'index.js'));
+  });
+
+  test('updates readme.markdown', async function() {
+    await documentation(['readme index.js -s API'], { cwd: d });
+    const outputPath = path.join(d, 'readme.markdown');
+    expect(fs.readFileSync(outputPath, 'utf-8')).toMatchSnapshot();
+  });
+});
+
 describe('readme command', function() {
   const fixtures = path.join(__dirname, 'fixture/readme');
   const sourceFile = path.join(fixtures, 'index.js');

src/get-readme-file.js

@@ -5,14 +5,11 @@ const path = require('path');
 
 module.exports = function findReadme(dir: string) {
   const readmeFilenames = [
-    'README',
     'README.markdown',
     'README.md',
-    'README.txt',
     'Readme.md',
     'readme.markdown',
-    'readme.md',
-    'readme.txt'
+    'readme.md'
   ];
 
   const readmeFile = fs.readdirSync(dir).find(function(filename) {