Update README.md

Swarm 1.2.4: Fix minor bugs in algorithm for d=1, based on cppcheck r…

…esults

SWARM 2.1.1: Fix a bug with the fastidious option

Updated version number in swarm.h

Add a verbose mode. No more data sent to stderr by default.

swarm_breaker.py now has executable permission

Update the documentation (v1.2.6)

POPCNT cpu feature not required

Change of option name and version number

Added information about changes in version 1.2.3