Add Macports and MinGW instructions.

README.md

@@ -33,9 +33,14 @@ You can also download the official windows binaries on the
 
 ### macOS
 
+Homebrew:
 ```sh
 brew install libavif
 ```
+MacPorts:
+```sh
+sudo port install libavif
+```
 
 ### Linux
 
@@ -48,6 +53,13 @@ Red Hat-based distributions:
 sudo yum -y install libavif
 ```
 
+### MinGW
+
+For the "default" MSYS2 UCRT64 environment:
+```sh
+pacman -S mingw-w64-ucrt-x86_64-libavif 
+```
+
 ## Build Notes
 
 Building libavif requires [CMake](https://cmake.org/).