Replace README.txt with README.md

I applied the MR !2, MR failed.
Got the below error:

2.diff.patch:20: trailing whitespace.

2.diff.patch:22: trailing whitespace.

2.diff.patch:29: trailing whitespace.

warning: 3 lines add whitespace errors.

MR !1 donot have any update so hiding it.
MR !2 not following README.md template →

ravi kant → changed the visibility of the branch 3320675-replace-readme.txt-with to hidden.

Updated MR !2 according readme template suggestions.

Reviewed MR!2, few changes are required here:
1. Please remove "Introduction" from "## Table of contents"

Updated MR !2 according QA comment.

Reviewed MR!2, the file is updated as per the readme.md file template.
Hence marking it to RTBC.