Add documentation for notmuch insert --folder option.
.SH SYNOPSIS
.B notmuch insert
+.RI "[" options "]"
.RI "[ +<" tag> "|\-<" tag "> ... ]"
.SH DESCRIPTION
(it has same Message-ID), it will be added to the maildir folder and
notmuch database, but the tags will not be changed.
+Option arguments must appear before any tag operation arguments.
+Supported options for
+.B insert
+include
+.RS 4
+.TP 4
+.BI "--folder=<" folder ">"
+
+Deliver the message to the specified folder,
+relative to the top-level directory given by the value of
+\fBdatabase.path\fR.
+The default is to deliver to the top-level directory.
+
.RE
.SH EXIT STATUS