Added Link to Database Schema in README.md

[bksahu]

• Exported database schema in pdf format.
• Added link to the pdf in the README.md.

[tameeshB]

This was a much needed thing for this repository, the DB schema. The problem here is that, whenever we add DB schemas, we stick to .sql format so that someone else using the project can import the DB schema. There's no use exporting in PDF. Please add the .sql schema file to a 'migrations' directory on the project. Also, try to refrain from referring to external files as dependencies of open source projects, the third party service might go down any day, try to keep as much as you can inside the repository. Happy coding!