Add a comment on expectations in the "tar" target

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4179)

Configurations/unix-Makefile.tmpl

diff --git a/Configurations/unix-Makefile.tmpl b/Configurations/unix-Makefile.tmpl
index f56237c..fbe7e9a 100644
--- a/Configurations/unix-Makefile.tmpl
+++ b/Configurations/unix-Makefile.tmpl
@@ -679,6 +679,8 @@
 
 # Release targets (note: only available on Unix) #####################
 
+# If your tar command doesn't support --owner and --group, make sure to
+# use one that does, for example GNU tar
 TAR_COMMAND=$(TAR) $(TARFLAGS) --owner 0 --group 0 -cvf -
 PREPARE_CMD=:
 tar: