french translation

adding french version of index.md, colors.md, datum.md and geocoding.md
Update index.fr.md

Corrections mineures mise en page
Fix header
Fix spelling mistake
Corrections: mainly spelling with some rewording
Fix small spelling issues
Merge pull request #1 from ThomasG77/gh-pages

Relecture traduction fr
Merge pull request #96 from JeremyG/gh-pages

Add french translation