added part on using vald_linelist:

Carsus_atomic_data/Custom_atomic_data.ipynb

@@ -49,6 +49,33 @@
     "...\n",
     "```"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "### Using VALD linelists\n",
+    "\n",
+    "Possibly one of the most important reasons you would want to use a custom atomic data file would be to take advantage of a tailored VALD linelist. You can refer to [VALD's documentation](https://www.astro.uu.se/valdwiki) for information on these lists if you are unfamiliar. "
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "If you have included a VALD linelist in your atom_data file, then you **must** make the following change to your Yaml file for the linelist to be used:\n",
+    "``` yaml\n",
+    "...\n",
+    "line:\n",
+    "        disable: False\n",
+    "        broadening: [radiation, linear_stark, quadratic_stark, van_der_waals]\n",
+    "        vald_linelist:  #<-----\n",
+    "            use_linelist: True #<----- will default to false, so must set to True\n",
+    "            shortlist: \n",
+    "no_of_thetas: 20\n",
+    "...\n",
+    "```"
+   ]
   }
  ],
  "metadata": {