docs: add README files for experimental packages (#15933)

Author: jelbourn

src/cdk-experimental/README.md

@@ -0,0 +1,5 @@
+# Angular CDK Experimental
+
+This package contains prototypes and experiments in development for Angular CDK. Nothing in
+this package is considered stable or production ready. While the package releases with Angular
+CDK, breaking changes may occur with any release.

src/material-experimental/README.md

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+# Angular Material Experimental
+
+This package contains prototypes and experiments in development for Angular Material. Nothing in
+this package is considered stable or production ready. While the package releases with Angular
+Material, breaking changes may occur with any release.
+
+## Using the experimental components based on MDC Web
+Assuming your application is already up and running using Angular Material, you can add this
+component by following these steps:
+
+1. Install Angular Material Experimental & MDC WEB:
+
+   ```bash
+   npm i material-components-web @angular/material-experimental
+   ```
+
+2. In your `angular.json`, make sure `node_modules/` is listed as a Sass include path. This is
+   needed for the Sass compiler to be able to find the MDC Web Sass files.
+
+   ```json
+   ...
+   "styles": [
+     "src/styles.scss"
+   ],
+   "stylePreprocessorOptions": {
+     "includePaths": [
+       "node_modules/"
+     ]
+   },
+   ...
+   ```
+   
+3. Import the `NgModule` for the component you want to use. For example, the checkbox:
+```ts
+  import {MatCheckboxModule} from '@angular/material-experimental/mdc-checkbox';
+
+  @NgModule({
+    declarations: [MyComponent],
+    imports: [MatCheckboxModule],
+  })
+  export class MyModule {}
+```
+
+4. Add use the components just as you would the normal Angular Material components. For example,
+the checkbox: 
+```html
+  <mat-checkbox [checked]="isChecked">Check me</mat-checkbox>
+```
+
+5. Add the theme and typography mixins to your Sass. These align with the normal Angular Material
+mixins except that they are suffixed with `-mdc`. There is currently no pre-built CSS option for
+the experimental components. For example, using the checkbox:
+
+```scss
+  @import '~@angular/material/theming';
+  @import '~@angular/material-experimental/mdc-checkbox';
+  
+  $my-primary: mat-palette($mat-indigo);
+  $my-accent:  mat-palette($mat-pink, A200, A100, A400);
+  $my-theme:   mat-light-theme($my-primary, $my-accent);
+  
+  @include mat-checkbox-theme-mdc($my-theme);
+  @include mat-checkbox-typography-mdc();
+```