Add README.md

Makefile

@@ -4,9 +4,11 @@ AR     = $(CCPREFIX)ar
 AS     = $(CCPREFIX)as
 CC     = $(CCPREFIX)gcc
 LD     = $(CCPREFIX)ld
-RAKE   = rake
 RANLIB = $(CCPREFIX)ranlib
-QEMU   = qemu-system-i386
+
+GRUB_MKRESCUE = grub-mkrescue
+RAKE          = rake
+QEMU          = qemu-system-i386
 
 ABS_REPO = $(shell pwd)
 
@@ -68,7 +70,7 @@ clean-libkernaux:
 	$(MAKE) -C $(LIBKERNAUX_DIR) distclean || true
 
 $(IMAGE): $(GRUBCFG) $(MRUBYVISOR)
-	grub-mkrescue $(ROOTFS_DIR) -o $@
+	$(GRUB_MKRESCUE) $(ROOTFS_DIR) -o $@
 
 $(MRUBYVISOR): $(LIBKERNAUX) $(LIBMRUBY)
 	$(MAKE) -C $(SRC_DIR) mrubyvisor.multiboot2 CCPREFIX='$(CCPREFIX)' DEST='$(ABS_REPO)/$(DEST_DIR)' MRUBY_FLAGS='$(MRUBY_FLAGS)'

README.md

@@ -0,0 +1,23 @@
+mrubyvisor
+==========
+
+* **mrubyvisor** is **mruby** running in supervisor (kernel) mode.
+* **mruby** is a lightweight implementation of **Ruby**.
+* **Ruby** is an interpreted, dynamically typed programming language.
+
+
+
+Dependencies
+------------
+
+Edit the variable `CCPREFIX` in `Makefile` to point to the
+[cross-compiler for i386](https://wiki.osdev.org/GCC_Cross-Compiler).
+
+### Debian
+
+The list may be incomplete.
+
+* `build-essential`
+* `grub-common`
+* `qemu-system-x86`
+* `rake`