Add comment describing the need to have the runtime library compiled

with CHARGEN_RELOC enabled.

cfg/atarixl-largehimem.cfg

@@ -1,3 +1,8 @@
+# This config file provides a single big upper memory block (HIDDEN_RAM).
+# To achieve this, it relocates the character generator from $E000 to CHARGEN.
+# The runtime library must be compiled with CHARGEN_RELOC for this config
+# file to work. See libsrc/atari/Makefile.inc.
+
 FEATURES {
     STARTADDRESS: default = $2400;
 }