Added zipcodestack.com

[zipcodestack]

• [x] My submission is formatted according to the guidelines in the contributing guide
• [x] My addition is ordered alphabetically
• [x] My submission has a useful description
• [x] The description does not have more than 100 characters
• [x] The description does not end with punctuation
• [x] Each table column is padded with one space on either side
• [x] I have searched the repository for any relevant issues or pull requests
• [x] Any category I am creating has the minimum requirement of 3 items
• [x] All changes have been squashed into a single commit

[marcelscruz]

Thank you!