Added contribution doc

Author: whizsid

CONTRIBUTING.md

@@ -0,0 +1,22 @@
+# Contribution Guideline
+
+Please make sure your code matching with the ESLint rules and prettier
+rules by running the commands `npm run all:lint:check` and `npm run
+all:prettier;check`. Otherwise run the
+`npm run all:lint:fix` and `npm run all:prettier:fix` commands to fix
+automatically fixable issues. Fix other issues manually.
+
+## Project Structure
+
+This project contains two libraries and 3 example projects.
+
+- `packages/google-analytics-embed-react`:- React library
+- `packages/google-analytics-embed-types`:- Type definitions library
+- `sites/express-backend`:- ExpressJS server which providing the Google
+Backend authentication
+- `sites/react-backend-auth`:- ReactJS example project which showing the
+backend authentication.
+- `sites/react-frontend-auth`:- ReactJS example project which showing
+the frontend authentication.
+
+Please check the whether both examples working before submitting a PR.