Update CONTRIBUTING.md (#2377)
Update CONTRIBUTING.md (#2377)

Given a recent comment on a rejected Pull Request I updated the 
Contributing Guidelines to explicitely state that adding a new icon in a 
Pull Request (without creating an issue first) is subject to the same 
requirements as an icon request by itself.
danleech/simpleicons.org,simple-icons/simple-icons,simple-icons/simple-icons,danleech/simple-icons,danleech/simpleicons.org