Add minor improvements to CONTRIBUTE.md
Add minor improvements to CONTRIBUTE.md

As stated in #66.

ableco/css-modal,shawinder/css-modal,ableco/css-modal,0111001101111010/css-modal,joerodrig3/css-modal,shawinder/css-modal,joerodrig3/css-modal,drublic/css-modal,drublic/css-modal