added new requirements

requirements.txt

@@ -10,6 +10,7 @@ django-phonenumber-field==6.3.0
 django-phonenumbers==1.0.1
 django-sendgrid-v5==1.2.0
 djangorestframework==3.13.1
+drf-writable-nested==0.6.4
 drf-yasg==1.20.0
 drfpasswordless==1.5.8
 idna==3.3