Added directory creation step to instructions in README.md

[daviddever]

First off thanks, these are great additions, was a little surprised that it wasn't using the system time to take advantage of NTP to begin with.

Just ran into a small issue was that after cloning the repository make failed since the .../DisplayNixie/binfolder didn't exist.

pi@nixie:~/NixieClockRaspberryPi/DisplayNixie/src$ make
Building target: DisplayNixie
Invoking: Cross G++ Linker
g++  -o "../bin/DisplayNixie"  DisplayNixie.o   -lwiringPi
/usr/bin/ld: cannot open output file ../bin/DisplayNixie: No such file or directory
collect2: error: ld returned 1 exit status

So I added a step to the instructions in README.md to create the bin directory.

[UberEclectic]

Good catch! Thanks!