Describe installing symfony with custom stability

Author: mateuszsip

README.md

@@ -14,3 +14,11 @@ To choose specific symfony version define `SYMFONY_VERSION` env variable for fir
 Like:
 
 `SYMFONY_VERSION=3.4 docker-compose up -d`
+
+To install symfony with different stability (more about stability [here](https://getcomposer.org/doc/04-schema.md#minimum-stability)) define `STABILITY` env variable for first run.
+
+Like:
+
+`STABILITY=dev docker-compose up -d`
+
+will install symfony from master branch.