[PR] Update README with instructions on how to build flow.js
[PR] Update README with instructions on how to build flow.js

Summary:
Add instructions on how to build `flow.js`.

While trying to build the flow website (which I'm still trying) I notice that it expects the `flow.js` file inside the `bin` folder.

This PR adds instructions on how to achieve that.
Closes https://github.com/facebook/flow/pull/3825

Differential Revision: D4976475

Pulled By: gabelevi

fbshipit-source-id: 5c9e49811a9cb82f43c89a486686de3f29d6d3c1

gabelevi/flow,samwgoldman/flow,facebook/flow,jamesgpearce/flow,AgentME/flow,nmote/flow,AgentME/flow,MichaelDeBoey/flow,facebook/flow,mroch/flow,popham/flow,fletcherw/flow,JonathanUsername/flow,MichaelDeBoey/flow,nmote/flow,JonathanUsername/flow,nmote/flow,facebook/flow,claudiopro/flow,ylu1317/flow,claudiopro/flow,popham/flow,fletcherw/flow,nmote/flow,MichaelDeBoey/flow,mroch/flow,gabelevi/flow,samwgoldman/flow,TiddoLangerak/flow,facebook/flow,TiddoLangerak/flow,samwgoldman/flow,TiddoLangerak/flow,facebook/flow,ylu1317/flow,claudiopro/flow,mroch/flow,JonathanUsername/flow,samwgoldman/flow,claudiopro/flow,fletcherw/flow,jamesgpearce/flow,mroch/flow,MichaelDeBoey/flow,fletcherw/flow,JonathanUsername/flow,ylu1317/flow,JonathanUsername/flow,popham/flow,popham/flow,MichaelDeBoey/flow,jamesgpearce/flow,popham/flow,popham/flow,nmote/flow,AgentME/flow,samwgoldman/flow,nmote/flow,samwgoldman/flow,claudiopro/flow,ylu1317/flow,gabelevi/flow,AgentME/flow,gabelevi/flow,jamesgpearce/flow,gabelevi/flow,JonathanUsername/flow,jamesgpearce/flow,fletcherw/flow,jamesgpearce/flow,mroch/flow,JonathanUsername/flow,facebook/flow,mroch/flow,AgentME/flow,gabelevi/flow,popham/flow,TiddoLangerak/flow,ylu1317/flow,TiddoLangerak/flow,nmote/flow,gabelevi/flow,jamesgpearce/flow,fletcherw/flow,mroch/flow,samwgoldman/flow,facebook/flow