fix(setup): Include MailHog setup instructions

With the changes to passwordless account creation, contributors are
unable to create an account locally. MailHog spins up a local SMPT
server that can catch all outgoing emails, allowing contributors to
successfully create accounts and see any other emails generated locally.

Closes #15838

server/datasources.development.js

@@ -0,0 +1,18 @@
+module.exports = {
+    mail: {
+        connector: 'mail',
+        transport: {
+            type: 'smtp',
+            host: 'localhost',
+            secure: false,
+            port: 1025,
+            tls: {
+                rejectUnauthorized: false
+            }
+        },
+        auth: {
+            user: 'test',
+            pass: 'test'
+        }
+    }
+};