Add documentation for KMSDRM on *BSDs

docs/README-kmsbsd.md

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+KMSDRM on *BSD
+==================================================
+
+KMSDRM is supported on FreeBSD and OpenBSD. DragonFlyBSD is not tested yet, but should work. NetBSD isn't supported yet because the application will crash when creating the KMSDRM screen.
+
+WSCONS support has been brought back, but only as an input backend. It will not be brought back as a video backend to ease maintenance.
+
+OpenBSD note: Note that the video backend assumes that the user has read/write permissions to the /dev/drm* devices.
+
+
+===================================================
+SDL2 WSCONS input backend features
+===================================================
+1. It is keymap-aware; it will work properly with different keymaps.
+2. It has mouse support.
+3. Accent input is supported.
+4. Compose keys are supported.
+5. AltGr and Meta Shift keys work as intended.
+
+==================================================
+Partially working or no input on OpenBSD/NetBSD.
+==================================================
+
+The WSCONS input backend needs read/write access to the /dev/wskbd* devices, without which it will not work properly. /dev/wsmouse must also be read/write accessible, otherwise mouse input will not work.
+
+==================================================
+Partially working or no input on FreeBSD.
+==================================================
+
+The evdev devices are only accessible to the root user by default. Edit devfs rules to allow access to such devices. The /dev/kbd* devices are also only accessible to the root user by default. Edit devfs rules to allow access to such devices.