docs(input): add overview file (#2307)

src/lib/input/OVERVIEW.md

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+# md-input-container
+
+Inputs are the basic input component of Material 2. The spec can be found
+[here](https://www.google.com/design/spec/components/text-fields.html). `md-input-container` acts as
+a wrapper for native `input` and `textarea` elements that is used to add material design styles and
+behavior. The native element wrapped by the `md-input-container` must be marked with the `md-input`
+directive.
+
+<!-- example(input-overview) -->
+
+## Usage
+
+### `input` and `textarea` attributes
+
+All of the attributes that can be used with normal `input` and `textarea` elements can be used on
+elements within the `md-input-container` as well. This includes angular specific ones such as
+`ngModel` and `formControl`.
+
+The only limitations are that the `type` attribute can only be one of the values supported by
+`md-input-container` and the native element cannot specify a `placeholder` attribute if the
+`md-input-container` also contains a `md-placeholder` element. 
+
+#### Supported `input` types
+
+The following [input types](http://www.w3schools.com/TAGs/att_input_type.asp) can be used with
+`md-input-container`:
+* date
+* datetime-local
+* email
+* month
+* number
+* password
+* search
+* tel
+* text
+* time
+* url
+* week
+
+### Placeholder
+
+A placeholder is an indicative text displayed in the input zone when the input does not contain
+text. When text is present, the indicative text will float above this input zone.
+
+The `floatingPlaceholder` attribute of `md-input-container` can be set to `false` to hide the
+indicative text instead when text is present in the input.
+
+A placeholder for the input can be specified in one of two ways: either using the `placeholder`
+attribute on the `input` or `textarea`, or using an `md-placeholder` element in the
+`md-input-container`. Using both will raise an error.
+
+### Prefix and Suffix
+
+HTML can be included before, and after the input tag, as prefix or suffix. It will be underlined as
+per the Material specification, and clicking it will focus the input.
+
+Adding the `md-prefix` attribute to an element inside the `md-input-container` will designate it as
+the prefix. Similarly, adding `md-suffix` will designate it as the suffix.
+
+### Hint Labels
+
+Hint labels are the labels that show below the underline. An `md-input-container` can have up to two
+hint labels; one on the `start` of the line (left in an LTR language, right in RTL), and one on the
+`end`.
+
+Hint labels are specified in one of two ways: either using the `hintLabel` attribute of
+`md-input-container`, or using an `md-hint` element inside the `md-input-container`, which takes an
+`align` attribute containing the side. The attribute version is assumed to be at the `start`.
+Specifying a side twice will result in an exception during initialization.
+
+### Divider Color
+
+The divider (line under the `input` content) color can be changed by using the `dividerColor`
+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.