Rename exercises dir

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

topics/shell/solutions/files_size.md

@@ -0,0 +1,17 @@
+## Files Size
+
+### Objectives
+
+1. Print the name and size of every file and directory in current path
+
+Note: use at least one for loop!
+
+### Solution
+
+```
+#!/usr/bin/env bash
+
+for i in $(ls -S1); do
+    echo $i: $(du -sh "$i" | cut -f1)
+done 
+```