Rename exercises dir

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

topics/shell/solutions/num_of_args.md

@@ -0,0 +1,26 @@
+## Number of Arguments
+
+### Objectives
+
+* Write a script that will print "Got it: <argument value>" in case of one argument
+* In case no arguments were provided, it will print "Usage: ./<program name> <argument>"
+* In case of more than one argument, print "hey hey...too many!"
+
+### Solution
+
+```
+#!/usr/bin/env bash
+
+set -eu
+
+main() {
+  case $# in
+    0) printf "%s" "Usage: ./<program name> <argument>"; return 1 ;;
+    1) printf "%s" "Got it: $1"; return 0 ;;
+    *) return 1 ;;
+  esac
+}
+
+main "$@"
+```
+