Update changelog for 1.2.0.
Update changelog for 1.2.0.

mumer92/javapoet,ponsonio/javapoet,mumer92/javapoet,studyfordream/javapoet,kenzierocks/javapoet,square/javawriter,square/kotlinpoet,ks94/javapoet,sitexa/javapoet,ponsonio/javapoet,rajatguptarg/javapoet,auke-/javawriter,gk5885/javawriter,sormuras/javapoet,sitexa/javapoet,kenzierocks/javapoet,gk5885/javawriter,rajatguptarg/javapoet,square/kotlinpoet,ks94/javapoet,square/javapoet,auke-/javawriter,sormuras/javapoet,studyfordream/javapoet