Create configuring mail documentation

The configuration mail documentation is based on the online version at:
https://docs.cachethq.io/docs/configuring-mail

docs/setup/configuring-mail.md

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+# Configuring Mail
+
+Your `.env` file will need to include the following setting keys.
+
+```
+MAIL_DRIVER=smtp
+MAIL_HOST=mailtrap.io
+MAIL_PORT=587
+MAIL_USERNAME=null
+MAIL_PASSWORD=null
+MAIL_ADDRESS=null
+MAIL_NAME=null
+MAIL_ENCRYPTION=tls
+```
+
+After making changes to your mail configuration you'll need to run the
+following command within your Cachet installation directory.
+
+```bash
+$ php artisan config:cache
+
+# If you experience any issues after running this command, run this too:
+$ rm -rf bootstrap/cache/*
+```
+
+## Cachet + Mailgun
+
+Create an account with [Mailgun][1].
+
+MAIL\_USERNAME should be Mailgun domain.
+MAIL\_PASSWORD should be Mailgun API Key.
+
+Edit your `.env` file with the following variables.
+
+```
+MAIL_DRIVER=mailgun
+MAIL_HOST=smtp.mailgun.org
+MAIL_PORT=587
+MAIL_USERNAME=alt-three.com
+MAIL_PASSWORD=xxxx
+MAIL_ADDRESS=support@alt-three.com
+MAIL_NAME="Alt Three Services Limited"
+MAIL_ENCRYPTION=tls
+```
+
+The tls encryption setting is required to have e-mails be properly delivered.
+
+## Cachet + Spark Post
+
+Create an account with [Spark Post][2].
+
+Edit your `.env` file with the following variables.
+
+```
+MAIL_DRIVER=smtp
+MAIL_HOST=smtp.sparkpostmail.com
+MAIL_PORT=587
+MAIL_USERNAME=SMTP_Injection
+MAIL_PASSWORD=API_TOKEN
+MAIL_ADDRESS=support@alt-three.com
+MAIL_NAME="Alt Three Services Limited"
+MAIL_ENCRYPTION=tls
+```
+
+[1]: https://mailgun.com/
+[2]: https://www.sparkpost.com/