Add slugify as an extra requirement

Author: codingjoe

README.md

@@ -8,7 +8,7 @@ Syntax](https://docs.python.org/3/library/string.html#format-string-syntax).
 ### Installation
 
 ``` sourceCode bash
-pip install django-dynamic-filenames
+pip install django-dynamic-filenames[slugify]
 ```
 
 ### Samples

setup.cfg

@@ -21,6 +21,10 @@ keywords =
     django-storages
     file
 
+[extras]
+slugify =
+    unicode-slugify
+
 [pbr]
 skip_authors = true
 skip_changelog = true