Rename exercises dir

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

topics/shell/solutions/factors.md

@@ -0,0 +1,20 @@
+## Shell Scripting - Factors
+
+### Objectives
+
+Write a script that when given a number, will:
+
+* Check if the number has 2 as factor, if yes it will print "one factor"
+* Check if the number has 3 as factor, if yes it will print "one factor...actually two!"
+* If none of them (2 and 3) is a factor, print the number itself
+
+### Solution
+
+```
+#!/usr/bin/env bash
+
+(( $1 % 2 )) || res="one factor"
+(( $1 % 3 )) || res+="...actually two!"
+
+echo ${res:-$1}
+```