Add a CACHEDIR.TAG file to tmp/ (matomo-org#13653)

Note that this does not handle cases where users delete the whole tmp/
directory. However, it is to be expected that users depending on
that file would take care not to delete it.

Fixes matomo-org#12754

Author: literalplus

.gitignore

@@ -34,6 +34,7 @@ php_errors.log
 /robots.txt
 /tmp/*
 !/tmp/.gitkeep
+!/tmp/CACHEDIR.TAG
 /vendor/
 /.cache
 /.externalToolBuilders

tmp/CACHEDIR.TAG

@@ -0,0 +1,4 @@
+Signature: 8a477f597d28d172789f06886806bc55
+# This file is a cache directory tag created by Matomo.
+# For information about cache directory tags, see:
+#	http://www.bford.info/cachedir/