docs: fix installation instruction (#372)

Install instructions in the README.md fails with an error: „Could not find a version that satisfies the requirement graphene-sqlalchemy>=3“

This is because v3 is in beta. Therefore, installing with '--pre' fixes the problem.

Author: zahrevsky

Diff for: README.md

@@ -18,7 +18,7 @@ A [SQLAlchemy](http://www.sqlalchemy.org/) integration for [Graphene](http://gra
 For installing Graphene, just run this command in your shell.
 
 ```bash
-pip install "graphene-sqlalchemy>=3"
+pip install --pre "graphene-sqlalchemy"
 ```
 
 ## Examples