Merge pull request #78 from amardeshbd/CONTRIBUTING-README-MD

[ADDED] Contribution guideline

Author: hossain-khan

CONTRIBUTING.md

@@ -0,0 +1,15 @@
+Contribution Guideline
+=====================
+
+You are more than welcome to enhance, fix any demo you see in the app. A good way to start is look into existing [issues](https://github.com/amardeshbd/android-constraint-layout-cheatsheet/issues) and see 
+if any of them interests you.
+
+Development Guide
+----------------------
+It is important to keep consistency in the project. Here are some of the key things you need to keep in mind:
+
+* Follow existing app architecture for previewing demo
+    * Use `LayoutPreviewBaseActivity` if the demo does not need special user interaction
+    * or, Create subclass of `LayoutPreviewBaseActivity` with user interaction specific binding. See `LayoutVisibilityGoneActivity` as an example.
+* Follow existing resource naming convention. See [README](https://github.com/amardeshbd/android-constraint-layout-cheatsheet/blob/master/app/src/main/res/layout/.README.md)
+* Create vector assets and save it to [raw resources](https://github.com/amardeshbd/android-constraint-layout-cheatsheet/tree/master/resources/vector) for later use. See [README](https://github.com/amardeshbd/android-constraint-layout-cheatsheet/blob/master/resources/vector/README.md) for example resource and how to create new resource.