Another anonymous angel, inspired by a widow who slipped her wedding rings into a red Salvation Army kettle, has followed in the woman’s footsteps, quietly donating a diamond-encrusted cross to help the needy and to thank the people of Boston.The donor, who the Salvation Army believes is a woman, put a white envelope with the cross and an unsigned note in a kettle outside a Stop & Shop in Brookline last Saturday.In the note, the donor said she had moved from Australia to Brookline in April with her young son to escape a difficult life.