Apply sandbox badge (#47)

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.

Author: fdegir

README.md

@@ -17,6 +17,7 @@
 
 <img src="./images/logo.png" alt="Eiffel Shared Java" width="350"/>
 
+[![Sandbox badge](https://img.shields.io/badge/Stage-Sandbox-yellow)](https://github.com/eiffel-community/community/blob/master/PROJECT_LIFECYCLE.md#stage-sandbox)
 [![Build Status](https://travis-ci.org/eiffel-community/eiffel-commons.svg?branch=master)](https://travis-ci.org/eiffel-community/eiffel-commons)
 [![Codacy Badge](https://api.codacy.com/project/badge/Grade/d22bd08ce36a4f43b8d21ede2848ce81)](https://www.codacy.com/app/eiffel-commons-maintainers/eiffel-commons?utm_source=github.com&amp;utm_medium=referral&amp;utm_content=eiffel-community/eiffel-commons&amp;utm_campaign=Badge_Grade)
 [![Codacy Badge](https://api.codacy.com/project/badge/Coverage/d22bd08ce36a4f43b8d21ede2848ce81)](https://www.codacy.com/app/eiffel-commons-maintainers/eiffel-commons?utm_source=github.com&utm_medium=referral&utm_content=eiffel-community/eiffel-commons&utm_campaign=Badge_Coverage)