[doc] Add readme

README.md

+maintenanceMode
+===============
+
+Set you LimeSurvey instance on maintenance mode, for any purpose, update a survey, update instance ....
+
+## Usage
+
+In the plugin settings : you find different settings, the most important are the Date / time for maintenance mode. Just set a date with time here : the website are close to public.
+
+## Installation
+
+See [Install and activate a plugin for LimeSurvey](http://extensions.sondages.pro/install-and-activate-a-plugin-for-limesurvey.html)
+
+You need [renderMessage](https://framagit.org/SondagePro-LimeSurvey-plugin/renderMessage) plugin.
+
+### Via GIT
+- Go to your LimeSurvey Directory
+- Clone in plugins/maintenanceMode directory
+
+### Via ZIP dowload
+- Download <http://extensions.sondages.pro/IMG/auto/maintenanceMode.zip>
+- Extract : `unzip maintenanceMode.zip`
+- Move the directory to plugins/ directory inside LimeSUrvey
+
+## Translation
+
+The plugin use po file for translation, if you need a transation in your language : please, make a pull request.
+The file must be in locale/yourlanguagecode/messages.po, a modele.po is on locale directory.
+
+## Home page & Copyright
+- HomePage <http://extension.sondages.pro/>
+- Copyright © 2017 Denis Chenu <http://sondages.pro>
+
+Distributed under [AFFERO GNU GENERAL PUBLIC LICENSE Version 3](http://www.gnu.org/licenses/agpl.txt) licence.
+If you need a more permissive Licence [contact](http://extensions.sondages.pro/about/contact.html).