Add co.js

co.md

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+---
+title: co
+---
+
+[co]: https://github.com/tj/co
+[thunkify]: https://github.com/visionmedia/node-thunkify
+[unyield]: https://github.com/MatthewMueller/unyield
+
+[co] allows you to use generators to manage async flow.
+
+### Generator → promise
+
+A generator can `yield` a thunk or promise. Using `co()` will immediately invoke the block inside it.
+
+```js
+co(function * () {
+  yield Promise.resolve(true)
+}).then(...)
+```
+
+### Generator() → Promise()
+
+Use `co.wrap()`. Most of the time, you'll be using co.wrap.
+
+```js
+var fn = co.wrap(function * (val) {
+  return yield Promise.resolve(val)
+})
+
+fn().then(...)
+```
+
+### Generator() → Node callback()
+
+Use [unyield]. (You can [thunkify] this later)
+
+```js
+var get = unyield(function * () {
+})
+
+get(function (err, res) { ... })
+```
+
+### Node callback() → Thunk()
+
+Use [thunkify]. You can yield this.
+
+```js
+var readFile = thunkify(fs.readFile)
+
+co(function * () {
+  var data = yield readFile('index.txt', 'utf-8')
+})
+```