You can add automatically a signature at the end of your messages as a footer.
1. Go to: Settings > Signatures & addresses
2. Enter the signature that you want to display at the end of your message as a footer by filling in the field: Signature
3. Your signature will automatically be displayed when you compose a new message.
Configure signature by aliases
If you have set aliases (addresses), select the email address you want to configure and fill out the field Signature. The setting is only applied to the e-mail address you have selected.