Added simple check to Python version in notebook.

- Including a few minor comments.

python/Basics.ipynb

    "source": [
     "### 1. Set environment parameters\n",
     "\n",
-    "Be sure to set `env_name` to the name of the Unity environment file you want to launch."
+    "Be sure to set `env_name` to the name of the Unity environment file you want to launch. Ensure that the environment build is in the `python/` directory."
    ]
   },
   {
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "### 2. Load dependencies"
+    "### 2. Load dependencies\n",
+    "\n",
+    "The following loads the necessary dependencies and checks the Python version (at runtime). ML-Agents (v0.3 onwards) requires Python 3."
    ]
   },
   {
    "source": [
     "import matplotlib.pyplot as plt\n",
     "import numpy as np\n",
+    "import sys\n",
-    "%matplotlib inline"
+    "%matplotlib inline\n",
+    "\n",
+    "print(\"Python version:\")\n",
+    "print(sys.version)\n",
+    "\n",
+    "# check Python version\n",
+    "if (sys.version_info[0] < 3):\n",
+    "    raise Exception(\"ERROR: ML-Agents (v0.3 onwards) requires Python 3\")"
    ]
   },
   {