Patch: Add multiboot-capability to documentation

[Dennis Guse]

Hello,

so far the multiboot support is not included in the documentation.
This small patch changes docs/Runtime_config.md to include multiboot.

I think adding how to use multiboot with Grub would be nice, but this
does not seem to fit to the content of Runtime_config.md.
Do you have any suggestions where to put it?
>

This example shows how to start SeaBIOS from Grub2 via multiboot.
SeaVGABIOS is passed as a module.

```
set root=(usb0,msdos1)
multiboot /bios.elf.bin
module /vgabios.bin name=vgaroms/seavgabios.bin
boot
```
>

Best,
---
Dennis Guse