Add note in src/CHANGESET about how to make ChangeSet file smaller
Add note in src/CHANGESET about how to make ChangeSet file smaller

bk: 38f50951JkD9FvuRS86VzLnKak1e_w
bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper