Signed-off-by: Andreu Gallofre <andreugallofre@gmail.com>

Change the label sizing in CONTRIBUTING.md

Change the label size and merging conditions to match the actual labels
and conditions used
Signed-off-by: Andreu Gallofre <andreugallofre@gmail.com>

Add requested changes

Add missing dot, removes duplicated information and change requirements
for merging size/L
Signed-off-by: Andreu Gallofre <andreugallofre@gmail.com>