Replace README.txt with README.md

There are some issue in the patch.

2.diff.txt:59: trailing whitespace.
    1. First of all install the `Gallery Slider module`.
2.diff.txt:60: trailing whitespace.
    2. Then add multiple images to `Article content type`.
2.diff.txt:61: trailing whitespace.
    3. `Flush your cache`.
2.diff.txt:63: trailing whitespace.
    5. After that go to display settings.
2.diff.txt:67: trailing whitespace.

warning: 5 lines add whitespace errors.