docs: add usage

Author: Almouro

README.md

@@ -2,14 +2,25 @@
 
 [![Build Status](https://app.travis-ci.com/bamlab/react-native-performance.svg?branch=master)](https://app.travis-ci.com/bamlab/react-native-performance)
 
-Only Android is supported for now.
+Only Android is supported for now. Add a 👍 on [this issue](https://github.com/bamlab/react-native-performance/issues/18) if you want to see iOS support.
 
 ![rn-perf-monitor](https://user-images.githubusercontent.com/4534323/151138734-dc9af3b1-1e96-4416-8abd-346597a4dbe8.gif)
 
 
+## Usage
+
+1. Disable *JS Dev Mode* in the settings (shake your device to open the development menu, then click settings)
+2. Click **Start Measuring**
+3. Do stuff in your app
+4. Check the score!
+
 This is how the score is calculated below, quite naively, but open to suggestions:
 <img width="1050" alt="plugin-score" src="https://user-images.githubusercontent.com/4534323/142174520-ab09e61c-9727-4969-8e1e-a9b688f9fd78.png">
 
+Note that:
+- the score depends on the device used. We advice using a lower-end device to accentuate performance issues.
+- the score depends on what you do on your app while measuring. If you do nothing, your score should (hopefully) be 100!
+
 ## Install
 
 ### Flipper