feat(avatar): avatar component rfc (#534)

* feat: avatar component rfc

* fix: provide more a11y info regarding name prop

* fix: removes srcFallback prop

rfcs/avatar-component.md

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+# Avatar Component
+
+## Usage
+
+### Basic usage
+
+```javascript
+import * as React from 'react';
+import {Avatar} from 'baseui/avatar';
+
+export default () => <Avatar name="Jane Doe" src="https://url-of-img.jpg" />;
+```
+
+### Advanced usage
+
+```javascript
+import * as React from 'react';
+import {Avatar} from 'baseui/avatar';
+
+export default () => (
+  <Avatar
+    name="Jane Doe"
+    overrides={{
+      Avatar: {border: '2px solid blue'}
+    }}
+    size="scale1000"
+    src="https://img-may-not-exist.jpg"
+  />
+);
+```
+
+## Exports
+
+* `Avatar`
+* `StatefulAvatar`
+* `StyledAvatar`
+
+## `Avatar` API
+
+* `name: string` - Required
+  Defines an alternative text description of the image.
+* `overrides: {}` - Optional
+  * `Avatar: ?React.ComponentType` component to use for Avatar image styling
+* `size: string` - Optional. Defaults to `scale1000`
+  Defines the width of the image. Should be an option from the $theme.sizing list otherwise, will use default.
+* `src: string` - Required
+  Image to display.
+
+## `StatefulAvatarContainer` API
+
+* All props of a `Avatar` component. Handles displaying fallback color if provided src image fails to load.
+
+## Presentational components props API
+
+These properties are passed to every presentational (styled) component that is exported:
+
+* `$size: string`
+  Sets img width. Provided value should map to an option in $theme.sizing. If not found, defaults to `scale1000`.
+
+## Accessibility
+
+Top level `name` prop will be set on the `<img>` element as its `alt` attribute.