Add Contribution Guidelines which replaces current wiki

CONTRIBUTING.md

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+# Contribution Guidelines
+
+Thanks for your interest in contributing to **E2E TensorFlow Lite Tutorials**!
+
+Please follow these steps when contributing:
+
+Step 1. (Optional) Propose a project idea with either a brief description or an overview page if you wish to add more context about it.   
+Step 2. (Optional) Create issues for requesting help, in general in these areas:
+
+* Model training
+* Model conversion to tflite (and add metadata) <-- tflite model can reside in a team members' GitHub, in your project repo or on TFHub. Please validate the tflite with metadata in the Android app especially if you are publishing it to TFHub for wider adoption.
+* Android app
+* iOS app
+* write an E2E tutorial 
+
+Step 3. If you are interested in helping out, please create a project repo and add the link to the E2E TFLite Tutorials README. Update project to "In Progress" section.  
+Step 4. After project is completed, move from "In progress " to "Completed" section on E2E TFLite Tutorials. And request with a PR to add your project to [awesome-tflite].
+
+When createing a PR, please make sure to:
+* Provide a meaningful title such as "Update README.md to [action]" and do not title it as "Update README.md".
+* Make sure there are no broken links.