refactor(chore): add instruction to remove build warnings

Author: ghiscoding-SE

README.md

@@ -38,6 +38,24 @@ Check out the [Releases](https://github.com/ghiscoding/Angular-Slickgrid/release
 - version `2.x.x` for Angular 7+ 
   - since version `2.11.0`, you can also change your build `target` to `ES2015` for modern browser.
 
+#### Build Warnings (Angular 8+)
+Since Angular 8+, a few warnings starting to appear in the while doing a Prod build and this was asked a few separate issues. These warnings are **fine**, the reason is because I dynamically load all the SlickGrid extensions [here](https://github.com/ghiscoding/Angular-Slickgrid/blob/master/src/app/modules/angular-slickgrid/extensions/extensionUtility.ts#L29) and depending on which flags you use in the grid options, with `require('slickgrid/plugins/slick....');` and Angular is complaining about the use of `require`.
+
+The warnings are **normal** so the best way to fix this, is to simply remove the warnings, all you have to do is to add a file named `ngcc.config.js` in your project source root with the following content (you can also see this [commit](https://github.com/ghiscoding/angular-slickgrid-demos/commit/1fe8092bcd2e99ede5ab048f4a7ebe6254e4bee0) which fixes the Angular-Slickgrid-Demos):
+```js
+module.exports = {
+  packages: {
+    'angular-slickgrid': {
+      ignorableDeepImportMatchers: [
+        /slickgrid\//,
+        /flatpickr/,
+        /jquery-ui-dist\//,
+      ]
+    },
+  }
+};
+```
+
 ### Fully Tested with [Jest](https://jestjs.io/) (Unit Tests) - [Cypress](https://www.cypress.io/) (E2E Tests)
 Angular-Slickgrid reached **100%** Unit Test Coverage, we are talking about ~10,000 lines of code (+2,600 unit tests) that are now fully tested with [Jest](https://jestjs.io/). There are also over 300 Cypress E2E tests to cover most UI functionalities.