Update README.md

[someguy9]

Added colorkit.co and removed broken link (colorspark)

Checklist

• 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.
• I followed Contribution Guidelines.