make_stars <- function(pval) {
  stars = ""
  if(pval <= 0.001)
    stars = "***"
  if(pval > 0.001 & pval <= 0.01)
    stars = "**"
  if(pval > 0.01 & pval <= 0.05)
    stars = "*"
  if(pval > 0.05 & pval <= 0.1)
     stars = "."
  stars
}

library(broom)
library(dplyr)

mtcars %>%
  lm(mpg ~ wt + qsec, .) %>%
  tidy() %>%
  mutate(signif = sapply(p.value, function(x) make_stars(x)))

         term  estimate std.error  statistic      p.value signif
1 (Intercept) 19.746223 5.2520617   3.759709 7.650466e-04    ***
2          wt -5.047982 0.4839974 -10.429771 2.518948e-11    ***
3        qsec  0.929198 0.2650173   3.506179 1.499883e-03     **