docs(license): notes on license header

Adds implementation notes on license banner checker so that new
developers can quickly understand how it works.

Refs carbon-design-system#4985.

packages/web-components/IMPLEMENTATION_NOTES.md

@@ -294,3 +294,10 @@ The develoment environment looks at `STORYBOOK_IBMDOTCOM_WEB_COMPONENTS_USE_RTL`
 Both of above use [RTLCSS](https://rtlcss.com) to generate the RTL version. RTLCSS has feature of [conrtol](https://rtlcss.com/learn/usage-guide/control-directives/)/[value](https://rtlcss.com/learn/usage-guide/value-directives/) directives, that `@carbon/ibmdotcom-web-components` codebase [utilize](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0/packages/web-components/src/components/masthead/masthead.scss#L347-L356).
 
 How to use the RTL version of CSS can be seen at [the usage documentation](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0/packages/web-components/docs/enable-rtl.md).
+
+## License header
+
+We ensure that our source code has appropriate licence header, with two mechanisms:
+
+1. The [CI task](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0/package.json#L25) that [checks if all source files have license headers](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0/gulp-tasks/lint.js#L25-L50).
+2. The [pre-commit hook](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0/.lintstagedrc#L4) that [checks if all staged source files have license headers](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0/package.json#L27). If the license year is found stale in the step, we [update it](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0/tasks/check-license.js#L46-L54) here.