added email configuration docs (#228)

var/docs/emails.md

@@ -0,0 +1,25 @@
+# Emails
+
+Kimai uses the [Swift MailerBundle](https://symfony.com/doc/current/email.html) for sending emails. 
+Please read their documentation, there are a lot of possible configuration settings that you might want to adapt to your needs.
+
+If not otherwise noted, all emails will be sent instantly (unless spooling is activated in `config/packages/swiftmailer.yaml`).
+
+## Activating email
+
+You have to adapt two settings in your `.env` [configuration file](configurations.md):
+
+- `MAILER_URL` - your smtp connection details for sending emails
+- `MAILER_FROM` - an application wide "from" address for all emails
+
+## All existing emails
+
+The following emails will be sent by Kimai:
+
+### Security related emails
+
+If you want to change the content of the emails, please have a look at the [FOSUserBundle config](../../config/packages/fos_user.yaml) 
+and its [documentation](https://symfony.com/doc/current/bundles/FOSUserBundle/emails.html).
+
+- Password reset 
+- Account approval (only if activated, see [User docu](users.md))