README: add something about system requirements

This is actually all bullshit. There are many factors that can ruin the
video playback experience (and most outside of our or the user's
control).

One thing that makes sense is that this declares incompatibility with
Windows XP (fixes #2473).

README.md

@@ -26,6 +26,21 @@ variety of video file formats, audio and video codecs, and subtitle types.
 
 Releases can be found on the [release list][releases].
 
+## System requirements
+
+- A not too ancient Linux, or Windows Vista or later, or OSX 10.8 or later.
+- A somewhat capable CPU. Hardware decoding might sometimes help if the CPU
+  is too slow to decode video realtime, but must be explicitly enabled with
+  the `--hwdec` option. On Windows, a CPU with SSE4 instruction set is required
+  to get decent hardware decoding performance.
+- A not too crappy GPU. mpv is not intended to be used with bad GPUs. There are
+  many caveats with drivers or system compositors causing tearing, stutter,
+  etc. On Windows, you might want to make sure the graphics drivers are
+  current, especially OpenGL. In some cases, ancient fallback video output
+  methods can help (such as `--vo=xv` on Linux), but this use is not
+  recommended or supported.
+
+
 ## Downloads
 
 
@@ -103,6 +118,7 @@ OSX.
 If you want to build a Windows binary, you either have to use MSYS2 and MinGW,
 or cross-compile from Linux with MinGW. See [Windows compilation][windows_compilation].
 
+
 ## FFmpeg vs. Libav