Mention changes in NEWS

Author: lionel-

NEWS.md

@@ -1,5 +1,30 @@
 # vdiffr (development version)
 
+This release includes two major changes:
+
+1. The internal SVG engine has been updated. This means you will have
+   to regenerate all existing snapshots.
+
+2. The snapshot management system now uses testthat (see
+   https://testthat.r-lib.org/articles/snapshotting.html). Most of the
+   R and javascript code has been removed from vdiffr as a consequence.
+   vdiffr now serves as a reproducible SVG generation engine for
+   testthat snapshots.
+   
+
+## Migration of existing snapshots
+
+There are two steps to update your snapshots to vdiffr 1.0.
+
+1. This step is optional. Install the github-only 0.4.0 version of
+   vdiffr with `remotes::install_github("r-lib/[email protected]")`. This
+   release only contains the new SVG engine. Review the snapshot
+   changes as usual with `vdiffr::manage_cases()`.
+
+2. Install vdiffr 1.0.0 from CRAN, delete the `tests/figs` directory,
+   run `devtools::test()`, and then `testthat::snapshot_review()`.
+
+
 # vdiffr 0.4.0
 
 This is a github-only release that is meant to help you migrate from