Finely chop shallots and mince garlic. In a bowl toss shallots and garlic with coriander. Discard husks from tomatillos and rinse tomatillos under warm water to remove stickiness. Chop tomatillos and add to shallot mixture, tossing to combine. Peel and pit avocados and cut into 3/4-inch pieces. Add avocados, lime juice, and salt and pepper to taste to tomatillo mixture, gently tossing until just combined. Salsa may be made 4 hours ahead and chilled, its surface covered with plastic wrap.