Add Mailgun configuration variables to .env + explaination

Author: natboehm

.env.sample

@@ -38,3 +38,14 @@ export GIT_REPO_CHECKOUT=./tmp/index-co
 # to the address `http://localhost:4200/authorize/github`.
 export GH_CLIENT_ID=
 export GH_CLIENT_SECRET=
+
+# Credentials for configuring Mailgun. You can leave these commented out
+# if you are not interested in actually sending emails. If left empty,
+# a mock email will be sent to a file in your local '/tmp/' directory.
+# If interested in setting up Mailgun to send emails, you will have
+# to create an account with Mailgun and modify these manually.
+# If running a crates mirror on heroku, you can instead add the Mailgun
+# app to your instance and shouldn't have to mess with these.
+# export MAILGUN_SMTP_LOGIN=
+# export MAILGUN_SMTP_PASSWORD=
+# export MAILGUN_SMTP_SERVER=