In ggplot2, how to add text by group in facet_wrap?

Here is an example:

library(ggplot2)
set.seed(112)
df<-data.frame(g=sample(c("A", "B"), 100, T),
           x=rnorm(100),
           y=rnorm(100,2,3),
           f=sample(c("i","ii"), 100, T))
ggplot(df, aes(x=x,y=y, colour=factor(g)))+
geom_point()+geom_smooth(method="lm", fill="NA")+facet_wrap(~f)

My question is how to add text like the second plot by group into the plot.