Add “Compatibility with older versions” section to NEWS

diff --git a/doc/NEWS.adoc b/doc/NEWS.adoc
index 18753705f0ef0d20deb924998f7ff90b5b623509..adc185e2cfa494038f45c77367f17e84f3404d98 100644 (file)
--- a/doc/NEWS.adoc
+++ b/doc/NEWS.adoc
@@ -6,8 +6,26 @@ Ccache 4.0
 Unreleased
 
 
-Tooling
-~~~~~~~
+Compatibility with older versions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+- The default location of the cache directory has changed to follow the XDG
+  base directory specification (read more below). This means that scripts can
+  no longer assume that the cache directory is `~/.ccache` by default. The
+  `CCACHE_DIR` environment variable still overrides the default location just
+  like before.
+
+- The cache directory structure has changed compared to previous versions (read
+  more below). This means that ccache 4.0 cannot share cache results with
+  earlier versions. It is however safe to run 4.0 and earlier versions against
+  the same cache directory; cache bookkeeping, statistics and cleanup are
+  backward compatible, with one minor exception: some statistics counters
+  incremented by ccache 4.0 won’t be visible when running “ccache -s” with an
+  older version.
+
+
+Changed tooling
+~~~~~~~~~~~~~~~
 
 - CMake is now used instead of Autoconf for build configuration.