Create DEV_ENVIRONMENT.md (#409)
Create DEV_ENVIRONMENT.md (#409)

Summary:
Adding a recommended workflow for those that don't have a good setup after conversation in https://github.com/facebook/flipper/pull/227

Just a starting point but I think it's better than nothing.
Pull Request resolved: https://github.com/facebook/flipper/pull/409

Reviewed By: danielbuechele

Differential Revision: D14798121

Pulled By: jknoxville

fbshipit-source-id: 721a4bc07720709d1c7cc9f2fa7db69984f4c37e

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