Update README.md file according to README.md template

The patch has following issues:
warning: README.md has type 100644, expected 100755
Readme.md file is as per the readme.md file template.

Updated the readme.md file to modal_block module .

Create MR for README.md file update.
Please review.

Updated the file as per Drupal README.md template.
Kindly review the changes.

1. Still getting the warning: warning: README.md has type 100644, expected 100755
2. Use ### prior to Usage as it is a sub-section of Configuration.

HI thakurnishant_06

I applied your updated MR !4 into my local and I confirmed that your MR applied cleanly and addressed the comment found below the comment #6.

Please look at the screenshot I attached for your reference

Thank you