sdk: rename README
'help' target fails not finding a file, so follow up on a change[2] made
as a fix for main README[1].
1. d0113711a31f ("README: port to 21st century")
2. 751486b31fd9 ("build: fix README.md reference after rename")
Signed-off-by: Tomasz Maciej Nowak <[email protected]>
Tomasz Maciej Nowak