Extending concrete Slab
Your best bet for a good finish relies on doing three things:


Making sure that the foundation for the extension is well compacted and onto good material
Cut back and scable the joint with the existing 10x10 slab so that there is a good bond when the new concrete is cast. 
Drill and fix dowel bars into the existing slab across the joint with the extension. 


The number and type of bars will vary depending on the load from the new shed but you should be fine with something like 8mm (3/8") bars at 200mm (8") centres. 

You should probably note that this will give you a full structural connection and so may be too much for your particular problem unless you want to be absolutely sure that the slab and the extension don't separate.