Data Sources: Added docs for Umbraco uCssClassName

Fixes #151

docs/data-sources/data-source--ucssclassname.md

@@ -4,23 +4,31 @@
 
 ### Data Sources
 
-
-> :rotating_light::rotating_light::rotating_light: **This documentation has not been written yet.** :rotating_light::rotating_light::rotating_light:
-> If you have a desire to contribute back to the Contentment package project, (and you have the time), please consider helping me write the documentation pages for the data-sources.
-> Any discussion, guidance or feedback can be had on issue #151.
-> https://github.com/leekelleher/umbraco-contentment/issues/151
-
-
 #### uCssClassName
 
 A homage to @marcemarc's bingo-famous [uCssClassNameDropdown](https://our.umbraco.com/packages/backoffice-extensions/ucssclassnamedropdown/) package!
 
+This data-source enables you to select the class names from a local CSS file, to populate the items of a compatible editor, e.g. [Data List](../editors/data-list.md).
+
 
 ##### How to configure the editor?
 
-_[TBC]_
+The configuration of the uCssClassName data-source has the following options:
+
+![Configuration Editor for uCssClassName](data-source--ucssclassname--configuration-editor-01.png)
+
+The first field is **Path to stylesheet**, here you can enter a local relative CSS file path.
+
+e.g. `~/umbraco/lib/font-awesome/css/font-awesome.min.css`
+
+The next field **Class Name Regex** is used to set the [Regular Expression (RegEx)](https://docs.microsoft.com/en-us/dotnet/standard/base-types/regular-expressions) pattern to match the class names in the CSS file. For the [FontAwesome](https://fontawesome.com/) example, this would be `\.fa-([^:]*?):before`.
+
+The next field is for **Exclusions** _(optional),_ a comma-separated list of partial class names to exclude from the resulting list.
+
+The last field is **Icon Pattern**, this is a formatted string to set the icon's class for inside the Umbraco backoffice. In the case of FontAwesome, this would be `icon-fa fa-{0}`.
 
 
 ##### What is the value's object-type?
 
-_[TBC]_
+The value for the uCssClassName data-source item is a `string`.
+Depending on the `List editor` used, this may be wrapped in a `List<string>`.