Documentation: Make it clear there is no OS firmware yet if ever

[OK2MOP]

Hi,
a documentation change should be made on the first lines of README.md clearly stating there is presently no opensource firmware for this radio. Because RADIODDITY started market this radio on their X account as well as on their pages (www[dot]radioddity[dot]com/products/baofeng-uv-5rm-plus) as having OpenSource firmware, linking this repository (unless the information leaked before the firmware actually got published, but I doubt that).

73, OK2MOP