Add description of important files not in VCS

README.md

@@ -1,4 +1,14 @@
 ### mail_error_notifs.py
 Script that can be used as a scheduled task to send email notifications when certain websites aren't available.
 
-Emails are sent using Sendgrid's service.
+Emails are sent using [Sendgrid](https://sendgrid.com)'s service.
+
+**Additional files used not in VCS:**
+- .env
+
+Specific or sensitive information is read from this file, such as Sendgrid's API key and the URL of target websites.
+
+- mail_error_notifs.log
+
+Where exceptions that occur during the execution of the script are logged to.
+The directory where the file should be stored is read from the environment variable LOGDIR, that can be set in the first file.