Add references to prerequisites

Users sometimes only read the OS-specific instructions, they must be
aware of the prerequisites.

README.md

@@ -35,7 +35,7 @@ Its features include:
  - [OTG mode](doc/hid-otg.md#otg)
  - and more…
 
-## Requirements
+## Prerequisites
 
 The Android device requires at least API 21 (Android 5.0).
 

doc/linux.md

@@ -61,6 +61,8 @@ _See [build.md](build.md) to build and install the app manually._
 
 ## Run
 
+_Make sure that your device meets the [prerequisites](/README.md#prerequisites)._
+
 Once installed, run from a terminal:
 
 ```bash

doc/macos.md

@@ -29,6 +29,8 @@ _See [build.md](build.md) to build and install the app manually._
 
 ## Run
 
+_Make sure that your device meets the [prerequisites](/README.md#prerequisites)._
+
 Once installed, run from a terminal:
 
 ```bash

doc/windows.md

@@ -38,6 +38,8 @@ _See [build.md](build.md) to build and install the app manually._
 
 ## Run
 
+_Make sure that your device meets the [prerequisites](/README.md#prerequisites)._
+
 Scrcpy is a command line application: it is mainly intended to be executed from
 a terminal with command line arguments.