added constraints into qp spline path optimizer doc
added constraints into qp spline path optimizer doc

xiaoxq/apollo,ApolloAuto/apollo,ycool/apollo,ycool/apollo,jinghaomiao/apollo,ycool/apollo,xiaoxq/apollo,wanglei828/apollo,jinghaomiao/apollo,ApolloAuto/apollo,xiaoxq/apollo,xiaoxq/apollo,ycool/apollo,jinghaomiao/apollo,ApolloAuto/apollo,jinghaomiao/apollo,ApolloAuto/apollo,xiaoxq/apollo,wanglei828/apollo,wanglei828/apollo,ycool/apollo,wanglei828/apollo,wanglei828/apollo,ApolloAuto/apollo,ApolloAuto/apollo,jinghaomiao/apollo,xiaoxq/apollo,ycool/apollo,wanglei828/apollo,jinghaomiao/apollo