Add news bullet

Author: hadley

NEWS.md

@@ -1,5 +1,7 @@
 # testthat (development version)
 
+* New `vignette("mocking")` explains mocking in more detail (#1265).
+* New `vignette("challenging-functions")` provides an index to other documentation based on the type of challenge you are facing (#1265).
 * When running a test interactively, testthat now reports the number of succeses. The results should also be more useful if you are using nested tests.
 * The hints generated by `expect_snapshot()` and `expect_snapshot_file()` now include the path to the package, if its not in the current working directory (#1577).
 * `expect_snapshot_file()` now clearly errors if the `path` doesnt exist (#2191).