zipmerge: document open file descriptor usage
Addresses #234.
diff --git a/man/zipmerge.mdoc b/man/zipmerge.mdoc
index 3ead20b..2b8a7c1 100644
--- a/man/zipmerge.mdoc
+++ b/man/zipmerge.mdoc
@@ -29,7 +29,7 @@
.\" OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN
.\" IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
-.Dd December 18, 2017
+.Dd March 17, 2021
.Dt ZIPMERGE 1
.Os
.Sh NAME
@@ -89,3 +89,10 @@
.An Dieter Baron Aq Mt dillo@nih.at
and
.An Thomas Klausner Aq Mt tk@giga.or.at
+.Sh CAVEATS
+.Nm
+uses one open file descriptor per zip archive.
+If you need to merge a lot of zip archives, check your shell's
+file descriptor ulimit and either increase it or run
+.Nm
+multiple times with e.g. 1000 source zip archives each time.
