Update CSFML version to 2.5.1

- Add macOS build script
- Update existing nuget build script

tools/nuget/README.md

@@ -12,12 +12,19 @@ All build scripts follow the same routine:
 
 * `build.win.ps1`
     * A [PowerShell](https://github.com/PowerShell/PowerShell) script to build Windows native libraries. By default, both `win-x86` and `win-x64` are built. To build only one specific Runtime Identifier, pass it to the script as a parameter (`./build.win.ps1 win-x64`)  
-    You must have Visual Studio 2017, cmake, and the Windows SDK 8.1 installed to run this script.
+    You must have Visual Studio 2019, cmake, and the Windows SDK 10 installed to run this script.
 
 * `build.linux.sh`
     * A [Bash](https://www.gnu.org/software/bash/) script to build libraries for a generic Linux distribution. The [Runtime Identifier](https://docs.microsoft.com/en-us/dotnet/core/rid-catalog) of the linux distribution must be specified as a parameter to the script.  
     This generic script doesn't install any of its dependencies. As such, you must have cmake and make installed, as well as all [SFML Dependencies](https://www.sfml-dev.org/tutorials/2.5/compile-with-cmake.php#installing-dependencies) available to cmake.
 
+* `build.macos.sh`
+    * A [Bash](https://www.gnu.org/software/bash/) script to build libraries for macOS. The [Runtime Identifier](https://docs.microsoft.com/en-us/dotnet/core/rid-catalog#macos-rids) of the macOS distribution must be specified as parameter to the script.
+    Currently supported RIDs are:
+        * osx.10.15-x64
+        * osx.11.0-x64
+        * osx.11.0-arm64 (requires SFML 2.6)
+
 * `build.docker.sh`, `build.docker.ps1`
     * A script to build the native libraries for a Linux distribution in a Docker container. If no RID is specified as a parameter, all supported RIDs will be built. Currently, the supported RIDs are:
     * `alpine-x64`