binaries: document min requirements for target systems

contrib/android/Readme.md

@@ -10,6 +10,8 @@ To generate an APK file, follow these instructions.
 ✓ _These binaries should be reproducible, meaning you should be able to generate
    binaries that match the official releases._
 
+- _Minimum supported target system (i.e. what end-users need): Android 6.0 (API 23)_
+
 This assumes an Ubuntu (x86_64) host, but it should not be too hard to adapt to another
 similar system.
 

contrib/build-linux/appimage/README.md

@@ -4,6 +4,8 @@ AppImage binary for Electrum
 ✓ _This binary should be reproducible, meaning you should be able to generate
    binaries that match the official releases._
 
+- _Minimum supported target system (i.e. what end-users need): x86_64, glibc 2.28_
+
 This assumes an Ubuntu host, but it should not be too hard to adapt to another
 similar system. The host architecture should be x86_64 (amd64).
 

contrib/build-wine/README.md

@@ -3,6 +3,8 @@
 ✓ _These binaries should be reproducible, meaning you should be able to generate
    binaries that match the official releases._
 
+- _Minimum supported target system (i.e. what end-users need): x86_64, Windows 10 (1809)_
+
 This assumes an Ubuntu (x86_64) host, but it should not be too hard to adapt to another
 similar system.
 

contrib/osx/README.md

@@ -4,6 +4,8 @@ Building macOS binaries
 ✓ _This binary should be reproducible, meaning you should be able to generate
    binaries that match the official releases._
 
+- _Minimum supported target system (i.e. what end-users need): macOS 11_
+
 This guide explains how to build Electrum binaries for macOS systems.