Update the README to show how to pull in HostOS correctly

Because HostOS is a dependency of this plugin, we need that pulled in. The
example sketch has that, but the README did not.

Fixes #10.

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

README.md

@@ -24,9 +24,11 @@ Using the extension is as simple as including the header, registering it with
 
 ```c++
 #include <Kaleidoscope.h>
+#include <Kaleidoscope-HostOS.h>
+#include <Kaleidoscope/HostOS-select.h>
 #include <Kaleidoscope-Unicode.h>
 
-KALEIDOSCOPE_INIT_PLUGINS(Unicode);
+KALEIDOSCOPE_INIT_PLUGINS(HostOS, Unicode);
 
 void setup() {
   Kaleidoscope.setup();