Make chicken in the slow cooker: Mix all the spices together in small bowl. Cover chicken thighs with spice rub. Coat bottom of slow cooker with sliced onions then top with chicken thighs. Place lime wedges between each thigh then pour beer and water over top. Cook on low for 6-8 hours or high for 3-4 hours. Remove chicken from the slow cooker then shred and place in separate bowl with a little extra liquid from the slow cooker. Discard the onions, limes, and remaining liquid.