Wash and zest the lime; put the zest in a measuring jug. Then juice the limes and add the juice to the measuring jug as well.
Slice the chiles in half, discard the seeds and stem, then chop both halves of the chiles up and add to lime juice. Mince the garlic and add as well. Then add the soy sauce and oil, stir.
Rinse the cooked shrimp, peel if necessary, and pop the shrimp into the dressing mixture to marinate for 15 minutes (you can let it marinate for up to several hours, if you wish: just refrigerate.)
Remove the shrimp from the dressing with a slotted spoon (or by hand) and set on a small plate for a minute.
Wash and dry the spinach leaves, then toss them in the dressing mixture that the shrimp were in. Put them in a salad bowl.
Wash and peel the avocados, remove and discard the pits, then slice and arrange the avocado slices, then the shrimp, on top of the spinach. (Note: only do this part when you are ready to serve, otherwise the avocado will brown on you. Don't toss after the avocado slices are on, or you may end up with a mooshy salad.)