docs(feature-targeting): Add README (#4533)

packages/mdc-feature-targeting/README.md

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+<!--docs:
+title: "Feature Targeting"
+layout: detail
+section: components
+excerpt: "Provides infrastructure to allow CSS styles to be included or excluded categorically."
+path: /catalog/feature-targeting/
+-->
+
+# Feature Targeting
+
+MDC Feature Targeting provides infrastructure to allow CSS styles to be included or excluded categorically.
+
+Most of the time, you shouldn't need to depend on `mdc-feature-targeting` directly. However, understanding it can be useful if you're interested in having more control over when certain types of MDC styles are emitted.
+
+## Installation
+
+```
+npm install @material/feature-targeting
+```
+
+## Basic Usage
+
+### Styles
+
+Authoring component styles:
+
+```scss
+@import "@material/feature-targeting/functions";
+@import "@material/feature-targeting/mixins";
+
+@mixin my-component-core-styles($query: mdc-feature-all()) {
+  $feat-structure: mdc-feature-create-target($query, structure);
+
+  @include mdc-feature-targets($feat-structure) {
+    // ...
+  }
+}
+```
+
+Consuming component styles:
+
+```scss
+@import "@material/feature-targeting/functions";
+@import "my-component-mixins";
+
+// To include all styles (using the default of mdc-feature-all() defined above):
+@include my-component-core-styles;
+
+// Or, to include a specific subset of styles:
+@include my-component-core-styles(structure);
+@include my-component-core-styles(mdc-feature-any(color, typography));
+// The above two @includes and the following @include would produce equivalent results:
+@include my-component-core-styles(mdc-feature-without(animation));
+```
+
+## Sass Mixins and Functions
+
+Mixin | Description
+--- | ---
+`mdc-feature-targets($feature-targets...)` | Conditionalizes content to only be emitted if the given feature target(s) is/are queried.
+
+Function | Description
+--- | ---
+`mdc-feature-create-target($feature-query, $targeted-feature)` | Returns a variable to be passed to `mdc-feature-targets` in order to filter emitted styles.
+`mdc-feature-all($feature-queries...)` | Returns a query object which will result in emitting `mdc-feature-targets` blocks that match _all_ of the specified features. Passing no arguments results in all blocks being emitted, which is the most common use case.
+`mdc-feature-any($feature-queries...)` | Returns a query object which will result in emitting `mdc-feature-targets` blocks that match _any_ of the specified features. Passing no arguments results in no blocks being emitted.
+`mdc-feature-without($feature-query)` | Returns a query object which will result in emitting `mdc-feature-targets` blocks that do _not_ match the specified feature.
+
+`$feature-query` and `$feature-queries` refer to one or more of the values listed below under Supported Features.
+
+### Supported Features
+
+MDC Web's styles are currently split according to the following features:
+
+* `structure` - All baseline styles that don't fit into any other category
+* `animation` - Styles responsible for causing animations and transitions to occur
+* `color` - Color-specific styles which rely on `mdc-theme` variables
+* `typography` - Typography-specific styles which rely on `mdc-typography`