Add Windows installation instructions to README

Can now be installed into game directory as sub-directory.

README.md

@@ -47,6 +47,32 @@ Finally install it to your system (optional),
 
     ninja -C build install
 
+Installing
+----------
+
+If building from source, run:
+
+    ninja -C build install
+
+to install xsystem4 on your system.
+
+### Windows
+
+The recommended way to install on Windows is to copy the xsystem4 directory
+into the game directory. E.g. if Sengoku Rance is installed at
+`C:\Games\AliceSoft\Sengoku Rance`, then your filesystem should look something
+like this:
+
+    C:\
+        Games\
+            AliceSoft\
+                Sengoku Rance\
+                    System40.ini
+                    xsystem4\
+                        xsystem4.exe
+                        ...
+                    ...
+
 Running
 -------
 
@@ -59,3 +85,8 @@ Alternatively, run xsystem4 from within the game directory,
 
     cd /path/to/game_directory
     xsystem4
+
+### Windows
+
+If you've installed xsystem4 into the game directory as described above, simply
+double-click `xsystem4.exe` to run the game.