added python-dotenv and updated crontab example to load environment variables so the scan_comics will get the correct settings.

cbreader/crontab

@@ -1 +1 @@
-*/15    *       *       *       *       python /src/manage.py scan_comics
+*/15    *       *       *       *       python /src/manage.py scan_comics --settings=cbreader.settings.base