Update contributing.md
Update contributing.md

Added a welcome message and copied the paragraph about contributing that is in the README. This way it looks a bit better in case someone stumbles upon this repo through https://github.com/Tatoeba/tatoeba2/contribute and jumps next into the contributing guidelines.
Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2