Add documentation

Author: willshowell

src/lib/input/input.md

@@ -85,3 +85,25 @@ The underline (line under the `input` content) color can be changed by using the
 attribute of `md-input-container`. A value of `primary` is the default and will correspond to the
 theme primary color. Alternatively, `accent` or `warn` can be specified to use the theme's accent or
 warn color.
+
+### Custom Error Matcher
+
+By default, error messages are shown when the control is invalid and the user has interacted with
+(touched) the element or the parent form has been submitted. If you wish to customize this
+behavior (e.g. to show the error as soon as the invalid control is dirty), you can use the
+`errorStateMatcher` property of the input container. To use this property, create a function in
+your component class that accepts an `NgControl` and returns a boolean. A result of `true` will
+display the error messages.
+
+```html
+<md-input-container [errorStateMatcher]="myErrorStateMatcher">
+  <input mdInput [(ngModel)]="myInput" required>
+  <md-error>This field is required</md-error>
+</md-input-container>
+```
+
+```ts
+function myErrorStateMatcher(control: NgControl): boolean {
+  return control.invalid && control.dirty;
+}
+```