Added OpenBSD instructions

Building-from-source.md

@@ -160,4 +160,22 @@ This Ubuntu PPA contains backports of Qt 6 to various older versions: https://la
 This unofficial CLI installer allows downloading and installing the latest first-party builds of Qt to your system
 (whether it works against your distribution may vary): https://github.com/miurahr/aqtinstall
 
+</details>
+
+<!---------------------------------------------------------------------------------------------------------------->
+<details>
+<summary><h2>OpenBSD</h2></summary>
+
+OpenBSD, and by extension these instructions, are not officially supported. YMMV.
+
+Firstly, install the required packages:
+```
+pkg_add cmake sdl2 qtbase
+```
+Then, follow these instructions:
+- Make the build directory: `mkdir build && cd build`
+- Export the Qt directory: `export Qt5_DIR=/usr/local/lib/qt5/cmake/Qt5`
+- Make CMake files: `cmake -DCMAKE_CXX_FLAGS='-I/usr/local/include -O2' -DCMAKE_EXE_LINKER_FLAGS='-z wxneeded' ..`
+- Make the executable: `make`
+
 </details>