rcar-gen3: README: Update release information
rcar-gen3: README: Update release information

This modifies the README file for the following update:
- Add information for dunfell-dev.

Signed-off-by: Khang Nguyen <8dfb9c771c579920706fa8a68e7ba00c68a28f22@renesas.com>
Signed-off-by: Takamitsu Honda <dd018642994e4b3e742377b6d773664f62294df8@renesas.com>
Change-Id: I80be54b8d5571ca6e3f36693527070bf20f6ed32

renesas-rcar/meta-renesas,renesas-rcar/meta-renesas,renesas-rcar/meta-renesas,renesas-rcar/meta-renesas