Rename exercises dir

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

topics/shell/solutions/empty_files.md

@@ -0,0 +1,20 @@
+## Empty Files
+
+### Objectives
+
+1. Write a script to remove all the empty files in a given directory (including nested directories)
+
+### Solution
+
+```
+#! /bin/bash
+for x in *
+do
+    if [ -s $x ]
+    then
+        continue
+    else
+        rm -rf $x
+    fi
+done
+```