Replace README.txt with README.md

This looks great!

Only the following line has an issue:

It Requires the library called [nicebooks](https://packagist.org/packages/nicebooks/isbn)

"Requires" should not start with a capital letter and the library is "nicebooks/isbn" instead of "nicebooks".

I would change it into:

This module requires the library ["nicebooks/isbn"](https://packagist.org/packages/nicebooks/isbn).
If you installed this module with Composer using the command `composer require drupal/isbn`, it has been installed automatically.

Hi MegaChriz,

As you suggest the changes I added a commit for this MR please review it.
Thanks

Merged the code. Thanks all!

Automatically closed - issue fixed for 2 weeks with no activity.