cocos2d-x don't have glee20 branch anymore

[ayuer]

In Readme.md the cocos2d-x part, we have the following step:
$ git clone git://github.com/cocos2d/cocos2d-x.git
$ cd cocos2d-x
$ git checkout gles20

But the gles20 branch don't exists anymore.