added a bit more explaination to the slicing lab

Author: PythonCHB

source/exercises/slicing.rst

@@ -31,6 +31,20 @@ Your functions should look like:
   def exchange_first_last(seq):
       return a_new_sequence
 
+
+**Hint:**
+
+Your functions should work with ALL sequences. That means that you cannot use list methods, like ``.append``, because that won't work with strings and tuples. But all sequences support concatenation with the ``+`` operator.
+
+Item or Sequence?
+.................
+
+A key difference between using a single index: ``seq[i]`` and using a slice: ``seq[i:j], seq[:i], seq[i:]`` is that using an index returns a single item, whereas a slice always returns a sequence -- even if that sequence is of length one or even empty. And concatenation requires a sequence, so make sure you use slicing if you want to concatenate the results.
+
+.. note:: Python "gotcha" with strings. Python does not have a character type. Instead of a character, you get a length-one string. This can cause confusion sometimes, as other sequences return a single item when you index, so when you index into a list of numbers, you get number -- which is not a list (or any type of sequence). But with strings, when you index into (or loop through) a string, you get a length-one string, which is, itself a string, and therefor a valid sequence. So: ``a_string[i] + another_string`` works, but ``a_list[i] + another_list`` does not work.
+
+
+
 Tests:
 ------