doc: add the mobile.md doc

docs/mobile.md

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+# Experimental support for Chrome for Android
+
+You can try Playwright against Chrome for Android today. This support is experimental. Support for devices is tracked in the issue [#1122](https://github.com/microsoft/playwright/issues/1122).
+
+## Requirements
+
+- [Playwright 1.6](https://www.npmjs.com/package/playwright) or newer
+- [ADB daemon](https://developer.android.com/studio/command-line/adb) running and authenticated with your device.
+- [`Chrome 87`](https://play.google.com/store/apps/details?id=com.android.chrome) or newer installed on the device
+- "Enable command line on non-rooted devices" enabled in `chrome://flags`.
+
+ > Playwright will be looking for ADB daemon on the default port `5037`. It will use the first device available. Typically running `adb devices` is all you need to do.
+
+## How to run
+
+```js
+const { _clank } = require('playwright');
+
+(async () => {
+  const context = await _clank.launchPersistentContext('', {
+    viewport: null
+  });
+  const [page] = context.pages();
+  await page.goto('https://webkit.org/');
+  console.log(await page.evaluate(() => window.location.href));
+  await page.screenshot({ path: 'example.png' });
+  await context.close();
+})();
+```
+
+> [Clank](https://chromium.googlesource.com/chromium/src/+/master/docs/memory/android_dev_tips.md) is a code name for Chrome for Android.
+
+## Known limitations
+- Raw USB operation is not yet supported, so you need ADB.
+- Only `launchPersistentContext` works, launching ephemeral contexts is not supported.
+- Passing `viewport: null` is necessary to make sure resolution is not emulated.
+- Device needs to be awake to produce screenshots. Enabling "Stay awake" developer mode will help.
+- We didn't run all the tests against the device, so not everything works.