adding a 'usage' to the monitoring script

diff --git a/tools/racs_monitor.sh b/tools/racs_monitor.sh
index 398aafc..957120f 100755
--- a/tools/racs_monitor.sh
+++ b/tools/racs_monitor.sh
@@ -2,6 +2,13 @@

 # auxiliary script to monitor RAMdisk and memory utilization
 #
+# Usage:
+#	racs_monitor.sh <LOC>
+#
+# <LOC> is an optinal argument to specify the location of the working space
+#	indicated to use in the RACS core count_reads.sh pipeline; if it is not
+#	specified then the scrip will look into RAMDISK (/dev/shm) or TMP (/tmp)
+#
 # This script is part of
 #	 RACS v1.0 (2018/2019) -- Open source tools for Analizing ChIP-Seq data
 #