Build requirements to README.txt

[m2ger]

Might note in the README.txt that libbluetooth-dev is required for building:

$ make -j
gcc -c -o based.o based.c -Wall -MMD -O
gcc -c -o main.o main.c -Wall -MMD -O
main.c:1:33: fatal error: bluetooth/bluetooth.h: No such file or directory
 #include <bluetooth/bluetooth.h>
                                 ^
compilation terminated.
make: *** [main.o] Error 1
make: *** Waiting for unfinished jobs....

Which on Debian/Ubuntu based distros can be obtained by $ sudo apt-get install libbluetooth-dev