add note about pymc and bambi installation (#472)

docs/examples/observed_space_examples.ipynb

@@ -8,6 +8,20 @@
     "# Predictive elicitation"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "f06af29a",
+   "metadata": {},
+   "source": [
+    "PyMC and Bambi are optional dependencies of PreliZ, you only need them if you want to use the `predictive_explorer` function with PyMC or Bambi models.\n",
+    "\n",
+    "To install them, you can run the following command:\n",
+    "\n",
+    "```bash\n",
+    "conda install -c conda-forge pymc bambi\n",
+    "```"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": 2,
@@ -17,10 +31,12 @@
    "source": [
     "import arviz as az\n",
     "import preliz as pz\n",
-    "import pymc as pm\n",
-    "import bambi as bmb\n",
     "import numpy as np\n",
-    "import pandas as pd"
+    "import pandas as pd\n",
+    "\n",
+    "# Optional dependencies\n",
+    "import pymc as pm\n",
+    "import bambi as bmb"
    ]
   },
   {