add slugify info

Author: PrenSJ2

pages/custom-fields/custom-fields.md

@@ -11,4 +11,16 @@ You can find the machine name of a custom field by clicking on it in your
 [custom field settings](https://secure.tutorcruncher.com/setup/attrs/). See [creating a client](#3-create-a-client)
  for an example which sets a custom field for the Client's date of birth.
 
+If you are using dropdown custom fields, you must post the slugified value of the dropdown option. 
+For example, a value `Doesn't suit student` would be posted as `doesnt_suit_student`.
+
+<details>
+<summary>
+  What is slugification?
+</summary>
+
+When you convert spaces or repeated dashes to single dashes. Remove characters that aren't alphanumerics, underscores, or hyphens. Convert to lowercase. Also strip leading and trailing whitespace, dashes, and underscores.
+
+</details>
+