Add UXtweak to User Research Tools

[bustingbussy]

Checklist

• [x] I made something good today 💐.
• [x] I put links in the correct format: [Tool](link) — description.
• [x] My description starts with the lower-case letter and ends with the full stop.
• [x] I put the tool in the alphabetical order.

Optional

• [x] I added the appropriate labels: free, open-source or mac.
• [x] I put the tool only to one category.
• [x] I followed Contribution Guidelines.

[bustingbussy]

Sorry for making a few errors along the way, I hope it will be alright in the end. Hope to see the tool added soon! Thank you for such a great list!