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

DEV_ENVIRONMENT.md

@@ -0,0 +1,32 @@
+# Developer Environment Setup
+
+## Desktop app and Flipper Plugins (Javascript)
+
+The flipper desktop source uses Flow and ESLint. Feel free to use your preferred setup, but this is our recommended approach:
+
+**Editor**: Visual Studio Code
+
+**Installed Extensions**:
+ * ESLint
+ * Flow Language Support
+ 
+**Settings**:
+```json
+{
+    "flow.useNPMPackagedFlow": true,
+    "javascript.validate.enable": false,
+    "eslint.autoFixOnSave": true
+}
+```
+
+## Android plugins and SDK (Java)
+
+**Editor**: Android Studio
+
+## iOS plugins and SDK (Objective-C)
+
+**Editor**: XCode
+
+## Cross-platform SDK (Used by Android and iOS SDK) (C++)
+
+**Editor**: XCode, Android Studio, Visual Studio Code, Vim...