add example override for tag colour display

overrides/README.md

@@ -0,0 +1,11 @@
+# Customizing Paperless
+
+*See customization
+[documentation](https://paperless.readthedocs.io/en/latest/customising.html) 
+for more detail!*
+
+The example `.css` and `.js` snippets in this folder can be placed into
+one of two files in your ``PAPERLESS_MEDIADIR`` folder: `overrides.js` or 
+`overrides.css`. Please feel free to submit pull requests to the main 
+repository with other examples of customizations that you think others may
+find useful.

overrides/tag_colour_preview.js

@@ -0,0 +1,26 @@
+// The following jQuery snippet will add a small square next to the selection
+// drop-down on the `Add tag` page that will update to show the selected tag
+// color as the drop-down value is changed. Copy and paste it into
+// ``PAPERLESS_MEDIADIR/overrides.js`` to see the effects.
+
+let colour;
+let colour_num;
+
+colour_num = django.jQuery("#id_colour").val() - 1;
+colour = django.jQuery('#id_colour')[0][colour_num].text;
+django.jQuery('#id_colour').after('<div id="colour_square"></div>')
+
+django.jQuery('#colour_square').css({
+    'float': 'left',
+    'width': '20px',
+    'height': '20px',
+    'margin': '5px',
+    'border': '1px solid rgba(0, 0, 0, .2)',
+    'background': colour
+});
+
+django.jQuery('#id_colour').change(function () {
+    colour_num = django.jQuery("#id_colour").val() - 1;
+    colour = django.jQuery('#id_colour')[0][colour_num].text;
+    django.jQuery('#colour_square').css({'background': colour});
+});