Load raw C binary + docs

docs/SOFTWARE.md

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+# Writing software for PepperOS
+
+## Why would you want to do that?
+
+Honestly I have no idea. Maybe you have too much free time.  
+Keep in mind that the Pepper kernel is a personal project and it's full of bugs, inconsistencies, weird ways of doing things (and I don't care because it's my toy).  
+Now if you still want to write something for this OS, thank you. Follow along.
+
+## 1. Write the source code
+
+PepperOS is able to run programs written in x86 assembly, and C programs.
+
+### x86 Assembly
+
+Start your assembly file with the `bits 64` instruction, to emit 64-bit code.  
+You can add sections `.text`, `.data`, `.bss` as you need.  
+The three things to take in consideration here are:
+- PepperOS does not use the `syscall` instruction, instead it uses the old-fashioned `int 0x80` to trigger a system call.  
+- The entry point should be labelled as `_start`.
+- At the end of the file, there should be an exit system call followed by a loop, like so:
+
+```nasm
+.end:
+    mov rax, 0x3C
+    mov rdi, 0x0
+    int 0x80
+
+.loop:
+    jmp .loop
+```
+
+For an example, look at the file [pedicel.S](../user/pedicel.S).
+
+### C program
+
+You will find relevant headers in the `libc` directory. They contain system call wrappers, utility functions, and more. See what's implemented there and what's not.  
+To invoke a system call you can use the functions defined in `libc/syscall.h`.
+
+## 2. Add the Makefile rule and variable
+
+Now that your code is complete, add a Makefile rule to `user/Makefile` with your program name. You can just copy-paste the rule that applies to you (either from an Assembly source or C source) and change the name of the files (.raw, .elf, etc...) in the rule.  
+For clarity, raw binaries have the `.raw` extension, and ELF ones have `.elf`.  
+You also now have to add the name of the executable to the `USER_PROGRAMS` variable at the top of the global Makefile.  
+Finally, do `make user` to compile your program.
+
+## 3. Run your program
+
+You can now boot up PepperOS, in a VM or on real hardware, and use the kernel's shell to `list` files in the filesystem (to see if your executable was properly added), and then, run it with the `load` command. Congratulations, you made a program for a random hobby OS!
+
+## 4. (Optional) debugging
+
+Use GDB with the `make debug` rule!  
+For your information, user programs are loaded at `0x400000`. Can be good to know to set breakpoints.
+
+## 5. (Optional) contribute!
+
+If you like what you've done and you think it could be nice to add it to PepperOS, send it to me by e-mail: `xamidev (at) riseup (dot) net`. It may or may not be added in a future release... who knows?