Updated README-platforms.md

Also added platform specific README files

docs/README-bsd.md

@@ -0,0 +1,6 @@
+# FreeBSD / OpenBSD / NetBSD
+
+SDL is fully supported on BSD platforms, and is built using [CMake](README-cmake.md).
+
+If you want to run on the console, you can take a look at [KMSDRM support on BSD](README-kmsbsd.md)
+