List scrcpy characteristics in README

They were listed in the blog post introducing scrcpy:
<https://blog.rom1v.com/2018/03/introducing-scrcpy/>

README.md

@@ -6,6 +6,17 @@ It works on _GNU/Linux_, _Windows_ and _macOS_.
 
 ![screenshot](assets/screenshot-debian-600.jpg)
 
+It focuses on:
+
+ - **lightness** (native, displays only the device screen)
+ - **performance** (30~60fps)
+ - **quality** (1920×1080 or above)
+ - **low latency** ([35~70ms][lowlatency])
+ - **low startup time** (~1 second to display the first image)
+ - **non-intrusiveness** (nothing is left installed on the device)
+
+[lowlatency]: https://github.com/Genymobile/scrcpy/pull/646
+
 
 ## Requirements