Update documentation (#1)

Update documentation:

- Add links to blogpost about performance metrics
- Elaborate on RenderingMetrics format

README.md

@@ -17,6 +17,9 @@ Library supports collecting following performance metrics:
 - App Cold Startup Time
 - Rendering performance per Activity
 
+We recommend to read our blogpost ["Measuring mobile apps performance in production"](https://medium.com/booking-com-development/measuring-mobile-apps-performance-in-production-726e7e84072f) 
+first to get some idea on how performance metrics work and why those were chosen.
+
 ### Dependency
 
 The library is available on Maven Central:
@@ -26,7 +29,7 @@ implementation("com.booking:perfsuite:0.2")
 
 ### Collecting Startup Times
 
-Implement the callback invoked once Startup Time is collected:
+Implement the callback invoked once [Startup Time](https://medium.com/booking-com-development/measuring-mobile-apps-performance-in-production-726e7e84072f#e383) is collected:
 
 ```kotlin
 class MyStartupTimeListener : AppStartupTimeTracker.Listener {
@@ -57,7 +60,8 @@ class MyApplication : Application() {
 
 Implement the callback invoked every time when the foreground `Activity` is paused 
 (we can call it "the end of the screen session") and use `RenderingMetricsMapper` to
-represent rendering performance metrics in a convenient aggregated format:
+represent [rendering performance metrics](https://medium.com/booking-com-development/measuring-mobile-apps-performance-in-production-726e7e84072f#3eca) 
+in a convenient aggregated format:
 
 ```kotlin
 class MyFrameMetricsListener : ActivityFrameMetricsTracker.Listener {
@@ -85,7 +89,17 @@ class MyApplication : Application() {
 }
 ```
 
+As per the code sample above you can use `RenderingMetricsMapper` to collect frames metics in the aggreated format which is convenient for reporting to the backend.
+Then metrics will be represented as [`RenderingMetrics`](src/main/java/com/booking/perfsuite/rendering/RenderingMetrics.kt) instance, which will provide data on:
+- `totalFrames` - total amount of frames rendered during the screen session
+- `totalFreezeTimeMs` - total accumulated time of the UI being frozen during the screen session
+- `slowFrames` - amount of [slow frames](https://firebase.google.com/docs/perf-mon/screen-traces?platform=android#slow-rendering-frames) per screens session
+- `frozenFrames` - amount of [frozen frames](https://firebase.google.com/docs/perf-mon/screen-traces?platform=android#frozen-frames) per screens session
+
+Even though we support collecting widely used slow & frozen frames we [strongly recommend relying on `totalFreezeTimeMs` as the main rendering metric](https://medium.com/booking-com-development/measuring-mobile-apps-performance-in-production-726e7e84072f#2d5d)
+
 ## Additional documentation
+- [Measuring mobile apps performance in production](https://medium.com/booking-com-development/measuring-mobile-apps-performance-in-production-726e7e84072f) 
 - [App Startup Time documentation by Google](https://developer.android.com/topic/performance/vitals/launch-time)
 - [Rendering Performance documentation by Google](https://developer.android.com/topic/performance/vitals/render)
 - [Android Vitals Articles by Pierre Yves Ricau](https://dev.to/pyricau/series/7827)