Profiler API docs (#2176)

* First pass at Profiler API docs

content/docs/nav.yml

@@ -59,6 +59,8 @@
       title: Optimizing Performance
     - id: portals
       title: Portals
+    - id: profiler
+      title: Profiler
     - id: react-without-es6
       title: React Without ES6
     - id: react-without-jsx

content/docs/reference-profiler.md

@@ -0,0 +1,119 @@
+---
+id: profiler
+title: Profiler API
+layout: docs
+category: Reference
+permalink: docs/profiler.html
+---
+
+The `Profiler` measures how often a React application renders and what the "cost" of rendering is.
+Its purpose is to help identify parts of an application that are slow and may benefit from [optimizations such as memoization](https://reactjs.org/docs/hooks-faq.html#how-to-memoize-calculations).
+
+> Note:
+>
+> Profiling adds some additional overhead, so **it is disabled in [the production build](https://reactjs.org/docs/optimizing-performance.html#use-the-production-build)**.
+>
+> To opt into production profiling, React provides a special production build with profiling enabled.
+> Read more about how to use this build at [fb.me/react-profiling](https://fb.me/react-profiling)
+
+## Usage
+
+A `Profiler` can be added anywhere in a React tree to measure the cost of rendering that part of the tree.
+It requires two props: an `id` (string) and an `onRender` callback (function) which React calls any time a component within the tree "commits" an update.
+
+For example, to profile a `Navigation` component and its descendants:
+
+```js{3}
+render(
+  <App>
+    <Profiler id="Navigation" onRender={callback}>
+      <Navigation {...props} />
+    </Profiler>
+    <Main {...props} />
+  </App>
+);
+```
+
+Multiple `Profiler` components can be used to measure different parts of an application:
+```js{3,6}
+render(
+  <App>
+    <Profiler id="Navigation" onRender={callback}>
+      <Navigation {...props} />
+    </Profiler>
+    <Profiler id="Main" onRender={callback}>
+      <Main {...props} />
+    </Profiler>
+    </App>
+);
+```
+
+`Profiler` components can also be nested to measure different components within the same subtree:
+```js{2,6,8}
+render(
+  <App>
+    <Profiler id="Panel" onRender={callback}>
+      <Panel {...props}>
+        <Profiler id="Content" onRender={callback}>
+          <Content {...props} />
+        </Profiler>
+        <Profiler id="PreviewPane" onRender={callback}>
+          <PreviewPane {...props} />
+        </Profiler>
+      </Panel>
+    </Profiler>
+  </App>
+);
+```
+
+> Note
+>
+> Although `Profiler` is a light-weight component, it should be used only when necessary; each use adds some CPU and memory overhead to an application.
+
+## `onRender` Callback
+
+The `Profiler` requires an `onRender` function as a prop.
+React calls calls this function any time a component within the profiled tree "commits" an update.
+It receives parameters describing what was rendered and how long it took.
+
+```js
+function onRenderCallback(
+  id, // the "id" prop of the Profiler tree that has just committed
+  phase, // either "mount" (if the tree just mounted) or "update" (if it re-rendered)
+  actualDuration, // time spent rendering the committed update
+  baseDuration, // estimated time to render the entire subtree without memoization
+  startTime, // when React began rendering this update
+  commitTime, // when React committed this update
+  interactions // the Set of interactions belonging to this update
+) {
+  // Aggregate or log render timings...
+}
+```
+
+Let's take a closer look at each of the props:
+
+* **`id: string`** - 
+The `id` prop of the `Profiler` tree that has just committed.
+This can be used to identify which part of the tree was committed if you are using multiple profilers.
+* **`phase: "mount" | "update"`** -
+Identifies whether the tree has just been mounted for the first time or re-rendered due to a change in props, state, or hooks.
+* **`actualDuration: number`** -
+Time spent rendering the `Profiler` and its descendants for the current update.
+This indicates how well the subtree makes use of memoization (e.g. [`React.memo`](/docs/react-api.html#reactmemo), [`useMemo`](/docs/hooks-reference.html#usememo), [`shouldComponentUpdate`](/docs/hooks-faq.html#how-do-i-implement-shouldcomponentupdate)).
+Ideally this value should decrease significantly after the initial mount as many of the descendants will only need to re-render if their specific props change.
+* **`baseDuration: number`** -
+Duration of the most recent `render` time for each individual component within the `Profiler` tree.
+This value estimates a worst-case cost of rendering (e.g. the initial mount or a tree with no memoization).
+* **`startTime: number`** -
+Timestamp when React began rendering the current update.
+* **`commitTime: number`** -
+Timestamp when React committed the current update.
+This value is shared between all profilers in a commit, enabling them to be grouped if desirable.
+* **`interactions: Set`** -
+Set of ["interactions"](http://fb.me/react-interaction-tracing) that were being traced the update was scheduled (e.g. when `render` or `setState` were called).
+
+> Note
+>
+> Interactions can be used to identify the cause of an update, althoguh the API for tracing them is still experimental.
+>
+> Learn more about it at [fb.me/react-interaction-tracing](http://fb.me/react-interaction-tracing)

package.json

@@ -86,7 +86,8 @@
     "nit:examples": "prettier --config examples/.prettierrc --list-different \"examples/**/*.js\"",
     "prettier": "yarn format:source && yarn format:examples",
     "prettier:diff": "yarn nit:source && yarn nit:examples",
-    "reset": "rimraf ./.cache"
+    "reset": "rimraf ./.cache",
+    "start": "yarn dev"
   },
   "devDependencies": {
     "@babel/preset-flow": "^7.0.0",