Replace README.txt with README.md

The patch works fine and moving it to RTBC.

Note: Could observe the below message though while applying the patch:
3.patch:9: trailing whitespace.
The module provides a new Commerce Promotion type which adds extra items to an
3.patch:12: trailing whitespace.
You may use this module to create complex promotions like BOGO (Buy One,
3.patch:15: trailing whitespace.
Unlike Commerce's built-in Buy X Get Y promotion type, this module allows
3.patch:16: trailing whitespace.
adding free items to an order just with coupons, not depending on other
3.patch:19: trailing whitespace.
The advantage of this module that the same product is discounted. The
warning: 5 lines add whitespace errors.

ravi kant → made their first commit to this issue’s fork.

I have updated missing link in Requirements section.