I wasn't familiar with Kate Nash, the singer, songwriter, actor, and subject of Amy Goldstein’s “Underestimate the Girl,” before seeing this documentary. Now I find myself wanting to know more about her. She’s incredibly appealing, and that’s a big reason why the film is so enjoyable. It's a really good film that examines what it's like to be a musician today. This well-crafted film recounts Nash’s story, covering…