Rename exercises dir

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

topics/python/solutions/reverse_string.md

@@ -0,0 +1,16 @@
+## Reverse a String - Solution
+
+```
+my_string[::-1]
+```
+
+A more visual way is:<br>
+<i>Careful: this is very slow</i>
+
+```
+def reverse_string(string):
+    temp = ""
+    for char in string:
+        temp =  char + temp
+    return temp
+```