* Readme.md: Update supported platforms

Update Readme.md with the supported platforms in edk2-platforms:

* Added:
 - Arm: Juno, SGI
 - Hisilicon: D06, HiKey960
 - Intel: Comet Lake
 - Marvel: Armada 80x0, CN913, Solidrun MacchiatoBin
 - Raspberry Pi: RPi4

 * Removed:
  - Hisilicon: D02

* Updated
 - Marvell: Armada 70x0 (fixed broken link)

Signed-off-by: Samer El-Haj-Mahmoud <samer@elhajmahmoud.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>