Add chicken, water, coconut milk, tomatoes, garlic, ginger, chopped shallots, spices and seasonings ( except garam masala ) in the slow cooker/crockpot. Cook on low for 6 hours or high for 4 hours. Once the chicken is cooked, stir in chickpea and garam masala and cook for another 30 minutes. Add green chillies and chopped coriander just before serving.