README.transfer.rst: update instructions

Convert the file to reStructuredText markup. Fixes #39 (closed).

Rewrite the instructions where possible or otherwise update them. E.g. fix the changed boot loader paths, etc.

The "Manual formatting (BIOS only)", previosly "PC-BIOS (ISOHYBRID-MBR)", method is not tested and it is not clear if it still even works.