update changelog for 1.15.1 release
update changelog for 1.15.1 release

Vishwa1311/incubator-fineract,RanjithKumar5550/RanMifos,RanjithKumar5550/RanMifos,Vishwa1311/incubator-fineract,Vishwa1311/incubator-fineract,RanjithKumar5550/RanMifos