Rename exercises dir

Name it instead "topics" so it won't be
strange if some topics included "exercises" directory.

topics/shell/solutions/directories_comparison.md

@@ -0,0 +1,30 @@
+## Directories Comparison
+
+### Objectives
+
+1. You are given two directories as arguments and the output should be any difference between the two directories
+
+### Solution
+
+Suppose the name of the bash script is ```dirdiff.sh```
+
+```
+#!/bin/bash
+
+if test $# -ne 2
+then
+	echo -e "USAGE: ./dirdiff.sh directory1 directory2"
+	exit 1
+fi
+
+# check for the checksums. 
+# If both the checksums same, then both directories are same
+if test `ls -1 $1 | sort | md5sum | awk -F "  " '{print $1}'` == `ls -1 $2 | sort | md5sum | awk -F "  " '{print $1}'`
+then
+	echo -e "No difference between the 2 directories"
+	exit 0
+fi
+
+diff -q $1 $2
+
+```