Added reboot instruction to RPi section of README.md

[sfinktah]

If you don't reboot at some point after apt-get {update,upgrade} then the Makefiles in patches/... will incorrectly look for the pre-upgrade modules (4.19.57-v7+ on a fresh NOOBS build), rather than the upgraded ones -- as of right now, that would be 4.19.66-v7+

Linux raspberrypi 4.19.57-v7+ #1244 SMP Thu Jul 4 18:45:25 BST 2019 armv7l GNU/Linux
pi@raspberrypi:~/nexmon $ popd
~/nexmon/patches/bcm43455c0/7_45_154/nexmon
pi@raspberrypi:~/nexmon/patches/bcm43455c0/7_45_154/nexmon $ make

[...]

  APPLYING PATCHES gen/nexmon.mk => brcmfmac43455-sdio.bin (details: log/patches.log)
  BUILDING DRIVER for kernel 4.19 brcmfmac_4.19.y-nexmon/brcmfmac.ko (details: log/driver.log)
make[1]: *** /lib/modules/4.19.57-v7+/build: No such file or directory.  Stop.
make: *** [Makefile:52: brcmfmac.ko] Error 2

pi@raspberrypi:~/nexmon/patches/bcm43455c0/7_45_154/nexmon $ ls /lib/modules/
4.19.66+  4.19.66-v7+  4.19.66-v7l+