Add Gmail SMTP option + docs

backend/src/variables/smtp.ts

@@ -2,10 +2,12 @@ const SMTP_HOST_SENDGRID = 'smtp.sendgrid.net';
 const SMTP_HOST_MAILGUN = 'smtp.mailgun.org';
 const SMTP_HOST_SOCKETLABS = 'smtp.socketlabs.com';
 const SMTP_HOST_ZOHOMAIL = 'smtp.zoho.com';
+const SMTP_HOST_GMAIL = 'smtp.gmail.com';
 
 export {
     SMTP_HOST_SENDGRID,
     SMTP_HOST_MAILGUN,
     SMTP_HOST_SOCKETLABS,
-    SMTP_HOST_ZOHOMAIL
+    SMTP_HOST_ZOHOMAIL,
+    SMTP_HOST_GMAIL
 }