Add PyPI downloads badge to README (#237)

In the README file, a new badge has been added to show the number of PyPI downloads for the package. This change aims to provide users an easy way to see the popularity and widespread use of the drf-social-oauth2 package.

README.rst

@@ -7,6 +7,11 @@ Welcome to DRF-Social-OAuth2
 .. image:: https://www.codefactor.io/repository/github/wagnerdelima/drf-social-oauth2/badge
     :target: https://www.codefactor.io/repository/github/wagnerdelima/drf-social-oauth2/badge
 
+.. image:: https://static.pepy.tech/badge/drf-social-oauth2
+   :target: https://pepy.tech/project/drf-social-oauth2
+
+
+
 Installation
 ------------