add comment on division operator py3 vs py2

since these are lecture notes, and the students hear about python for
the first time (supposedly), it is very likely that they will use python
3 if the instructor is using python 3.
however, some of them might soon find themselves working on some remote
system which only has some python 2.x available.
therefore, my note.

Author: chichilalescu

Lecture-1-Introduction-to-Python-Programming.ipynb

@@ -1097,6 +1097,15 @@
      ],
      "prompt_number": 37
     },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "Note: The `/` operator always performs a floating point division in Python 3.x.\n",
+      "This is not true in Python 2.x, where the result of `/` is always an integer if the operands are integers.\n",
+      "to be more specific, `1/2 = 0.5` (`float`) in Python 3.x, and `1/2 = 0` (`int`) in Python 2.x (but `1.0/2 = 0.5` in Python 2.x)."
+     ]
+    },
     {
      "cell_type": "markdown",
      "metadata": {},