Add news bullet

Author: hadley

NEWS.md

@@ -1,5 +1,6 @@
 # testthat (development version)
 
+* New `SlowReporter` makes it easier to find the slowest tests in your package. The easiest way to run it is with `devtools::test(reporter = "slow")` (#1466).
 * Parallel testthat now does not ignore test files with syntax errors (#1360).
 * `expect_lt()`, `expect_gt()`, and friends have a refined display that is more likely to display the correct number of digits and shows you the actual values compared.
 * `describe()`, `it()`, and `test_that()` now have a shared stack of descriptions so that if you nest any inside of each other, any resulting failures will show you the full path.