Add a boot/README file explaining rustboot's organization a bit.

Author: graydon

src/Makefile

@@ -728,7 +728,7 @@ PKG_3RDPARTY := rt/valgrind.h rt/memcheck.h \
                 rt/uthash/uthash.h rt/uthash/utlist.h \
                 rt/bigint/bigint.h rt/bigint/bigint_int.cpp \
                 rt/bigint/bigint_ext.cpp rt/bigint/low_primes.h
-PKG_FILES := README \
+PKG_FILES := README boot/README \
              $(wildcard etc/*.*) \
              $(MKFILES) $(BOOT_MLS) boot/fe/lexer.mll \
              $(COMPILER_INPUTS) \

src/README

@@ -3,6 +3,7 @@ This is preliminary version of the Rust compiler.
 Source layout:
 
 boot/              The bootstrap compiler
+boot/README        - More-detailed guide to it.
 boot/fe            - Front end (lexer, parser, AST)
 boot/me            - Middle end (resolve, check, layout, trans)
 boot/be            - Back end (IL, RA, insns, asm, objfiles)