[ADDED] Some extra readme about the thumbnails.

Author: hossain-khan

resources/vector/README.md

@@ -2,3 +2,19 @@ Vector Images & Icons
 =========
 This is the container for RAW vector assets that is used in the project.
 Most of the assets are created by simple vector editor at [https://vectr.com](https://vectr.com).
+
+
+Layout Thumbnail Specification
+-------------------------------
+To have consistent look, use following spec for layout thumbnail.
+* Canvas size: `640px X 640px` with 2px `#000000` border.
+* Box color: `#00acc1` with no opacity.
+* Constraints color: `#006064` with 85% opacity, border: 1px `#000000` with no opacity.
+* When multiple views are required to show relational positioning, use colors that are close to original color.
+* Name assets using conventions defined in [README](https://github.com/amardeshbd/android-constraint-layout-cheatsheet/blob/master/app/src/main/res/layout/.README.md)
+* Use Android Studio to import the **`SVG`** image to Android Vector drawable. 
+
+Examples
+-----------
+* https://vectr.com/hossain/a11KCVsbcM.svg?width=640&height=640
+* https://vectr.com/hossain/dUHU9o4er.svg?width=640&height=640