Add CHANGELOG entry

CHANGELOG.md

@@ -50,6 +50,21 @@ For example:
 
 This was added in [pull request #2672: Add static spacing override classes](https://github.com/alphagov/govuk-frontend/pull/2672). Thanks to [Patrick Cartlidge](https://github.com/patrickpatrickpatrick) for this contribution.
 
+#### Change the Button component background colour
+
+For non-GOV.UK branded websites, you can now change the Button component background colour.
+
+To change the Button component background colour set the `$govuk-button-background-colour` Sass variable.
+
+```scss
+@import "node_modules/govuk-frontend/govuk/base";
+
+$govuk-button-background-colour: govuk-colour("blue");
+@import "node_modules/govuk-frontend/govuk/components/button/index";
+```
+
+This was added in [pull request #2752: Change the Button component background colour](https://github.com/alphagov/govuk-frontend/pull/2752). Thanks to [Nick Colley](https://github.com/NickColley) for this contribution.
+
 ### Deprecated features
 
 #### Remove deprecated `govuk-header__navigation--no-service-name` class in the header