Add -x and -h options to the script header doc (#272)

z.sh

@@ -24,6 +24,8 @@
 #     * z -l foo  # list matches instead of cd
 #     * z -e foo  # echo the best match, don't cd
 #     * z -c foo  # restrict matches to subdirs of $PWD
+#     * z -x      # remove the current directory from the datafile
+#     * z -h      # show a brief help message
 
 [ -d "${_Z_DATA:-$HOME/.z}" ] && {
     echo "ERROR: z.sh's datafile (${_Z_DATA:-$HOME/.z}) is a directory."