Apply sandbox badge (#34)

* Apply sandbox badge

This PR applies sandbox badge to the repository to signal
the level of maturity of the project as agreed by Eiffel
Bootstrap Technical Committee.

See https://github.com/eiffel-community/community/blob/master/PROJECT_LIFECYCLE.md#project-lifecycle-stages
for the details of project lifecycle.

See https://hackmd.io/SCImga0nS1qSh3QvsEOAVQ?view for the
discussion that took place during past Eiffel Bootstrap
Technical Committee Meetings.

* Update README.md

README.md

@@ -17,9 +17,11 @@
 
 <img src="./images/logo.png" alt="Eiffel RemRem" width="350"/>
 
-# Eiffel RemRem Shared
+[![Sandbox badge](https://img.shields.io/badge/Stage-Sandbox-yellow)](https://github.com/eiffel-community/community/blob/master/PROJECT_LIFECYCLE.md#stage-sandbox)
 [![](https://jitpack.io/v/eiffel-community/eiffel-remrem-shared.svg)](https://jitpack.io/#eiffel-community/eiffel-remrem-shared)
 
+# Eiffel RemRem Shared
+
 This repository contains shared functionality of the [eiffel-remrem](https://github.com/eiffel-community/eiffel-remrem) components.
 
 # About this repository