[readme] document Promise support; remove Promise-related alternatives

readme.markdown

@@ -21,6 +21,11 @@ test('timing test', function (t) {
         t.equal(Date.now() - start, 100);
     }, 100);
 });
+
+test('test using promises', async function (t) {
+    const result = await someAsyncThing();
+    t.ok(result);
+});
 ```
 
 ```
@@ -128,7 +133,6 @@ By default, uncaught exceptions in your tests will not be intercepted, and will
 ## other
 
 - CoffeeScript support with https://www.npmjs.com/package/coffeetape
-- Promise support with https://www.npmjs.com/package/blue-tape or https://www.npmjs.com/package/tape-promise
 - ES6 support with https://www.npmjs.com/package/babel-tape-runner or https://www.npmjs.com/package/buble-tape-runner
 - Different test syntax with https://github.com/pguth/flip-tape (warning: mutates String.prototype)
 - Electron test runner with https://github.com/tundrax/electron-tap