[Impeller] Add an FAQ.

impeller/README.md

@@ -179,6 +179,7 @@ To your `AndroidManifest.xml` file, add under the `<application>` tag:
 
 ## Documentation, References, and Additional Reading
 
+* [Frequently Asked Questions](docs/faq.md)
 * [Impellers Coordinate System](docs/coordinate_system.md)
 * [How to Setup Xcode for GPU Frame Captures with Metal.](docs/xcode_frame_capture.md)
 * [Learning to Read GPU Frame Captures](docs/read_frame_captures.md)

impeller/docs/faq.md

@@ -0,0 +1,12 @@
+# Frequently Asked Questions
+
+* How do you run `impeller_unittests` with Playgrounds enabled?
+  * Playgrounds in the `impeller_unittests` harness can be enabled in one of
+    three ways:
+    * Edit `gn args` directly and add `impeller_enable_playground = true`.
+    * Add the `--enable-impeller-playground` flag to your `./flutter/tools/gn`
+      invocation.
+    * Set the `FLUTTER_IMPELLER_ENABLE_PLAYGROUND` to `1` before invoking
+      `./flutter/tools/gn`. Only do this if you frequently work with Playgrounds
+      and don't want to have to set the flags manually. Also, it would be a bad
+      idea to set this environment variable on CI.