docs: add readme for experimental MDC-based input

The input itself is mostly just an implementation of
a custom form-field control. It does not provide any styles, and should
be used in combination with the MDC based `mat-form-field`.

Most of this is already captured in the readme for `mdc-form-field`, but
for consistency we add a readme for `mdc-input`.

Author: devversion

src/material-experimental/mdc-input/README.md

@@ -1 +1,63 @@
-This is a placeholder for the MDC-based implementation of input.
+This is a prototype of an alternate version of `MatInput` built on top of
+[MDC Web](https://github.com/material-components/material-components-web). This component is
+experimental and should not be used in production.
+
+## How to use
+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` and 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 experimental `MatInputModule` and add it to the module that declares your
+   component:
+
+   ```ts
+   import {MatInputModule} from '@angular/material-experimental/mdc-input';
+
+   @NgModule({
+     declarations: [MyComponent],
+     imports: [MatInputModule],
+   })
+   export class MyModule {}
+   ```
+
+4. Use `MatInput` in your component's template in combination with the MDC-based `mat-form-field`.
+   Similarly to the input module from `@angular/material`, the MDC-based input module exposes
+   the form-field module.
+   
+   ```html
+   <mat-form-field>
+     <mat-label>Full name</mat-label>
+     <input matInput>
+   </mat-form-field>
+   ``` 
+   
+5. Ensure color and typography styles for `@angular/material-experimental` are set up. The input
+   itself does not provide any styles. If an input is used inside of the MDC-based
+   `mat-form-field`, either use a custom theme with the `mat-form-field-theme-mdc` mixin, or use
+   a prebuilt theme from `@angular/material-experimental/mdc-theming/prebuilt`.
+
+## API differences
+
+The API of input matches the one from `@angular/material/input`. Simply replace imports to
+`@angular/material/input` with imports to `@angular/material-experimental/mdc-input`.