Replace readme.txt with readme.md file.

Closes #3452694

README.md

-CONTENTS OF THIS FILE
----------------------
-
-* Introduction
-* Requirements
-* Installation
-* Configuration
-* Maintainers
-
-
-INTRODUCTION
-------------
-
-Optional end date
+# Optional end date
 
 Make the end date in a Date range field (DateRangeItem) optional.
 
 An extra "Optional end date" checkbox is added to the Date range field type
 Storage settings. When the box is checked, the end date is no longer required.
 
-This will mimic the behavior of of optional end dates in Drupal 8.9.x. So when
+This will mimic the behavior of optional end dates in Drupal 8.9.x. So when
 8.9.x is ready, you should be able to uninstall this module after updating, and
 use the core implementation instead.
 
-See https://www.drupal.org/project/drupal/issues/2794481 for more information
-about the core implementation.
+See [Allow end date to be optional](https://www.drupal.org/project/drupal/issues/2794481)
+for more information about the core implementation.
+
+For a full description of the module, visit the [project page](https://www.drupal.org/project/optional_end_date).
+
+To submit bug reports and feature suggestions, or to track changes, use the [issue queue](https://www.drupal.org/project/issues/optional_end_date).
+
+
+## Table of contents
+
+- Requirements
+- Installation
+- Configuration
+- Maintainers
 
 
-REQUIREMENTS
-------------
+## Requirements
 
 This module requires no modules outside of Drupal core.
 
 
-INSTALLATION
-------------
+## Installation
 
-Install as you would normally install a contributed Drupal module. Visit:
-https://drupal.org/documentation/install/modules-themes/modules-8 for further
-information.
+Install as you would normally install a contributed Drupal module.
+For further information, see
+[Installing Drupal Modules](https://www.drupal.org/docs/extending-drupal/installing-drupal-modules).
 
 
-CONFIGURATION
--------------
+## Configuration
 
 You need to check the "Optional end date" setting for all existing and new
 daterange fields, where the end date is not required.
 This is done in the "Storage settings" form of the daterange fields.
 
 
-MAINTAINERS
------------
+## Maintainers
 
-* Birk (Birk) - https://www.drupal.org/u/birk
+- Birk - [Birk](https://www.drupal.org/u/birk)