[metro-visualizer] Add README and development script

[simonpfish]

Summary

A README was added to the metro-visualizer package with an overview of the tool and setup instructions.

Additionally, a simple script that spawns a metro server for testing the visualizer was added to the package.

Note: I just completed the CLA for the first time.

Test plan

Read through the README. Run the development script from the metro-visualizer root folder and point the browser at http://localhost:8080/visualizer to make sure that the server is spawned correctly.

[codecov-io]

Codecov Report

Merging #292 into master will not change coverage.
The diff coverage is n/a.

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[rafeca]

Thanks @simonpfish and sorry for the delay reviewing! I'm gonna import this into phabricator 😃

[facebook-github-bot]

rafeca has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[rtmalone]

Hi, I realize this is still open. but I'm trying to get the visualizer to work and this is the only place I have found with any instructions. I'm stuck at a place where I think I need to see whether a specific package is being included in the bundle. When I follow the instructions here and visit localhost:8081/visualizer I receive

Unable to resolve module `react` from `/Users/my/path/to/project/node_modules/metro- visualizer/src/app/index.js`: Module `react` does not exist in the Haste module map or in these 
directories: /Users/my/path/to/project/node_modules

That's untrue however. React is definitely in my node_modules as it is a dep of every react-native related package.

[simonpfish]

Hey! You might have to cd into the metro-visualizer package directory and run npm install or yarn there. Let me know if this fixes anything, I can look more into it if it doesn't.

[cpojer]

I rebased and I'm gonna land it now.

[facebook-github-bot]

@cpojer is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[roytan883]

@simonpfish follow the README, can't make it work. check #373
maybe you should write more clear about how to start it, and with which version of RN.
I start it with yarn run metro serve, but can't build bundle, always says "Module AccessibilityInfo does not exist in the Haste module map".
which version of RN can work with metro-visualizer ?