Add initial documentation of RNTesterPlatformTest

Summary:
Changelog: [RNTester][Internal] - Add README for RNTesterPlatformTest framework

This diff adds some initial documentation for `RNTesterPlatformTest` to give other pointers on how to contribute their own platform tests (both internally and externally).

Reviewed By: lunaleaps

Differential Revision: D37566632

fbshipit-source-id: c203045f79c3c5488fd0dcbb8077e9041bd62b0f

packages/rn-tester/js/examples/Experimental/PlatformTest/README.md

@@ -0,0 +1,96 @@
+# RNTester PlatformTest
+
+A barebones manual testing framework designed to work as a mechanism for recreating [Web Platform Tests](https://github.com/web-platform-tests/wpt) in React Native in order to verify their compliance with W3C specifications.
+
+## Usage
+
+Any new test case will start as a new React component which at the very least accepts a `harness` prop which will contain all of the framework's testing APIs. This component you write will be passed into the `component` prop of a `RNTesterPlatformTest` element whose other props are used for documenting your test. Rendering this `RNTesterPlatformTest` will look similar to something like this:
+
+```js
+function ExampleTestCase ({ harness }) { /* ... */ }
+
+<RNTesterPlatformTest
+  title="Example Test"
+  description="Imagine there's a detailed description of this example test here"
+  instructions={[
+    "This is the example test's first step",
+    "A second step",
+    "A third step",
+  ]}
+  component={ExampleTestCase}
+/>
+```
+
+
+As of writting this README there are 2 different types of tests that the `harness` prop provides:
+
+### `test(testcase: (TestContext) => void, testName: string)`
+
+This is a method to create "regular" test reminicent of other frameworks such as Jest. These are meant to be run imperatively, and while that means that they technically could work in a `useEffect` hook as a way to run the test "on mount" — it is instead recommended to try and keep these tests in callbacks instead. A good alternative to running the test on mount would be to instead put the test in a callback and render a "Start Test" button which executes the callback.
+
+The first argument is the closure in which you will run your test and make assertions. The assertions are contained in the `TestContext` object which is provided in the test closure's first argument and contains the following assertions:
+
+* `assert_true(a: boolean, description: string): void`
+* `assert_equals(a: any, b: any, description: string): void`
+* `assert_greater_than_equal(a: number, b: number, description: string): void`
+* `assert_less_than_equal(a: number, b: number, description: string): void`
+
+Here's what a basic/contrived example which verifies the layout of a basic view:
+
+```js
+const EXPECTED_WIDTH = 100;
+const EXPECTED_HEIGHT = 200;
+
+function BasicLayoutTestCase({harness}) {
+  const viewRef = useRef(null);
+
+  const runTest = useCallback(() => {
+    const view = viewRef.current;
+    if (view != null) {
+      view.measureInWindow(({width, height}) => {
+        harness.test(({assert_equals}) => {
+          assert_equals(
+            width,
+            EXPECTED_WIDTH,
+            `view's computed width should be ${EXPECTED_WIDTH}`,
+          );
+          assert_equals(
+            height,
+            EXPECTED_HEIGHT,
+            `view's computed width should be ${EXPECTED_HEIGHT}`,
+          );
+        }, "view's width and height are correct");
+      });
+    }
+  }, [harness]);
+
+  return (
+    <>
+      <View
+        ref={viewRef}
+        style={{width: EXPECTED_WIDTH, height: EXPECTED_HEIGHT}}
+      />
+      <Button title="Start Test" onPress={runTest} />
+    </>
+  );
+}
+```
+
+### `useAsyncTest(description: string, timeoutMs?: number): AsyncPlatformTest`
+
+This is a hook which can be used to represent tests that expect something to happen *some time* in the future. If the test isn't marked as "done" within a certain amount of time (10 seconds by default but can be optionally specified in the hook's second argument). This hook returns an object containing a `done` function which is used for marking the completion of the async test.
+
+Here's what a basic example would look like for verifying that `pointermove` events are emitted:
+
+```js
+function BasicPointerMoveTestCase({harness}) {
+  const testPointerMove = harness.useAsyncTest('pointermove event recieved');
+
+  return (
+    <View
+      style={{width: 100, height: 100, backgroundColor: 'black'}}
+      onPointerMove={() => testPointerMove.done()}
+    />
+  );
+}
+```