doc: mention WriteHeapDump in 1.3 release notes
doc: mention WriteHeapDump in 1.3 release notes

LGTM=r
R=khr, r
CC=golang-codereviews
https://codereview.appspot.com/103810044

pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme