Update the README for the meson build

Build instructions are certainly nicer now...
And let's rename it to README.md in the process so it renders nicely.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
2 jobs for wip/meson in 44 seconds (queued for 1 second)
Status Job ID Name Coverage
  Prep
passed #5860843
container-prep

00:00:11

 
  Build
passed #5860844
build

00:00:33