Change the README.txt to a markdown file and add documentation on what the...

Change the README.txt to a markdown file and add documentation on what the module does and how to configure it.

README.md

+# Contextual Menu Block
+
+This module provides a menu block that site builders can place so that
+end users can easily navigate a page hierarchy.
+
+This module does not support variable starting levels and/or depth or
+other configuration options. It provides links for the immediate parent and
+sibling menu items of the active menu item, or it provides links to its immediate
+children. It does not provide parent of parent links or child of child links.
+
+Most sites will prefer to use this module in conjunction with the
+[Menu Breadcrumb](https://www.drupal.org/project/menu_breadcrumb) module, which will afford visitors with a link to navigate up
+the menu hierarchy when this block does not render a parent link (i.e.
+when the active item has children).
+
+For a full description of the module, visit the
+[project page](https://www.drupal.org/project/admin_toolbar).
+
+Submit bug reports and feature suggestions, or track changes in the
+[issue queue](https://www.drupal.org/project/issues/search/admin_toolbar).
+
+## Table of contents
+
+- Requirements
+- Recommended modules
+- Installation
+- Configuration
+
+## Requirements
+
+This module does not require any additional modules.
+
+## Recommended modules
+
+[Menu Breadcrumb](https://www.drupal.org/project/menu_breadcrumb):
+Most sites will prefer to use this module in conjunction with the
+Menu Breadcrumb module, which will afford visitors with a link to navigate up
+the menu hierarchy when this block does not render a parent link (i.e. when
+the active item has children).
+
+## 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).
+
+## Configuration
+
+Place the "Contextual menu" block using the layout tool of your choice.
+Then select which menu you want to be used as a contextual menu.