Add run on repl.it badge to README

This pull request configures this repository to be run on Repl.it.      It adds a `.replit` configuration file and a Repl.it badge to the `README`.
     You can read more about running repos on Repl.it [here](https://docs.repl.it/repls/dot-replit), or view the Repl [here](https://repl.it/@pieromqwerty/Flaskex).

.replit

@@ -0,0 +1,2 @@
+language = "python3"
+run = "python app.py"

README.md

@@ -8,6 +8,7 @@
 ![Contributions welcome](https://img.shields.io/badge/contributions-welcome-orange.svg)
 [![License](https://img.shields.io/badge/license-MIT-blue.svg)](https://opensource.org/licenses/MIT)
 [![Codacy Badge](https://api.codacy.com/project/badge/Grade/ef2f8f65c67a4043a9362fa6fb4f487a)](https://www.codacy.com/app/RDCH106/Flaskex?utm_source=github.com&utm_medium=referral&utm_content=RDCH106/Flaskex&utm_campaign=Badge_Grade)
+[![Run on Repl.it](https://repl.it/badge/github/anfederico/Flaskex)](https://repl.it/github/anfederico/Flaskex)
 
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