added some docu and a design document

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+design principles
+-----------------
+
+Use strict ANSI C99. This means no inline-assembly. This means
+we have to write assembly routines if we need things not available
+in C (lgdt, in, out, etc.).
+
+Make sure it compiles with every ANSI C99 compiler (gcc, clang, tcc, ...).
+
+Use only very simple makefile rules. Make sure we could also just
+use a simple shell script to compile the kernel.
+
+Avoid implementation and using of unsafe C functions. Provide safe
+counterparts wherever possible. There are of course exceptions like
+scrolling the VGA buffer with a memmove.