readme.md file added

Closes #3353608

README.md

+# Entity Reference Number Widget
+
+Provides a widget for D8 core entity reference fields where the target
+is specified directly via the numeric entity ID.
+
+## Requirements
+
+This module requires no modules outside of Drupal core.
+
+## Installation
+
+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).
+
+
+## Maintainers
+
+- Derek Wright [dww](https://www.drupal.org/u/dww)
+