Replace README.txt with README.md

Hi

So I applied patch #3 into 8.x-1.4 version of the module, but it didn't replace the README.txt into README.md it just adds content inside the README.txt and the file's content doesn't follow the Drupal README.md template → hence moving this to needs work.

for convenience this is what the patch created:

## CONTENTS OF THIS FILE

 * Introduction
 * Installation
 * Use cases
## INTRODUCTION

This module allows site builders to create node with their custom nid. If the node with given nid is already exist a message will occur that "Nid already exists".

## INSTALLATION

 * Install as you would normally install a contributed Drupal module.
   See: https://www.drupal.org/node/895232 for further information.
## USE CASES :-

1. In case when you write any business logic on the basis of node id you can use this module.
2. When you transfer data from one instance to another you can set id according your choice.