fix: add documentation about locales for colour blocks

plugins/field-colour/README.md

@@ -280,6 +280,25 @@ colourBlend.installBlock({
 });
 ```
 
+#### Message files and locales
+
+The blocks in this package contain text that can be 
+[localized](https://developers.google.com/blockly/guides/create-custom-blocks/localize-blocks)
+into multiple languages. As of August 2024, the relevant messages are
+included in the core Blockly language files.
+
+If your blocks show `%{BKY_COLOUR_BLEND_TITLE}` or similar text instead
+of the expected text, make sure that you either:
+
+- Import the [default Blockly modules](https://developers.google.com/blockly/guides/configure/web/translations#import_blockly_default_modules),
+which includes the English langfiles, or
+- Explicitly [import a language](https://developers.google.com/blockly/guides/configure/web/translations#import_blockly_languages)
+and call `setLocale` before using these blocks.
+
+For more information on Blockly's approach to localization, see the 
+[Localize Blocks](https://developers.google.com/blockly/guides/create-custom-blocks/localize-blocks)
+developer guide.
+
 ### API Reference
 
 - `setColours`: Sets the colour options, and optionally the titles for the