13.20 and 13.00 support (#49)

* add 13.20 support

* add 13.00 support

* Update README.md

update readme to reflect new 13.00, 13.20 and 13.60 support.

shellcode_kernel/linker.ld

@@ -15,4 +15,5 @@ SECTIONS
         *(.bss)
         *(.bss.*)
     }
+    ASSERT(SIZEOF(.shell_code) <= (DEFINED(SHELLCODE_LIMIT) ? SHELLCODE_LIMIT : 0x100000), "shellcode exceeds kernel code cave")
 }