Update build instructions in README.
diff --git a/README.md b/README.md
index 5e9ba5e..61e5706 100644
--- a/README.md
+++ b/README.md
@@ -8,3 +8,60 @@
The build has been fixed to work with a current SDCC (3.4) but currently the OS
(contiki.bin) will crash right after loading. Some debugging will be needed.
+
+How to build it
+===============
+
+Requirements
+------------
+
+You will need a patched version of SDCC. The linker was modified to generate
+relocation information, so the PRG executables can be loaded anywhere in memory
+and relocated at runtime before starting them. Running Contiki without that on
+the CPC would be much less interesting, because it is nearly impossible to write
+position independant z80 code.
+
+A patch for SDCC 3.4.1 (from the current SVN sources) is provided. Get the
+sources using SVN or a nightly snapshot and apply the patch, then configure
+SDCC as usual.
+
+You can still use the generated version of SDCC for other projects. The only
+difference is the addition of the -h flag to the linker. When this flag is set,
+executables are generated with relocation information.
+
+You will also need cpcgs from the cpctools project.
+
+Steps
+-----
+
+Once the patched SDCC is installed, the process is rather simple:
+
+ cd contiki-cpc
+ make clean
+ make cpc
+ make programs
+
+This will generate a dsk image with contiki and the various programs.
+
+Be careful to always do things in this order. The "cpc" target compiles the
+contiki core, and generate a defines file which is then used to have the apps
+call contiki routines. However, when contiki is recompiled, stuff move in
+memory and all programs must be recompiled. This means you should always do
+a "make clean", until the dependencies are properly defined in the makefiles.
+
+How to use it
+=============
+
+Boot your CPC or emulator and insert the disk in drive A (drive B is currently
+not supported). Then from the BASIC prompt type
+
+ run"contiki
+
+Contiki will take over the space allocated to BASIC and run there. Addresses
+100-4500 are for the code and internal variables. Between 4500 and A7FF is the
+heap memory, which is dynamically allocated to the different programs you may
+want to run. The heap size depends on the Contiki program size (low address)
+and use of memory by the CPC firmware and expansion ROMs (high address).
+
+Contiki is firmware-friendly and uses the CPC firmware for all IO operations
+(disk access, screen drawing, etc).