Provide better instructions for increasing shared memory size on Mac, in order to support MIT-SHM

[dpeterc]

Normal Mac user might not be familiar with proper ways of increasing shared memory size, in order to support MIT-SHM extension, which makes program work with decent speed.

In the installation instructions for MacOS, instruct the user to create file /etc/sysctl.conf with the following contents: kern.sysv.shmmax=134217728 kern.sysv.shmmin=1 kern.sysv.shmmni=1024 kern.sysv.shmseg=256 kern.sysv.shmall=32768 and reboot the system after creating the file. You could also provide a script to be executed with sudo permissions, which would create such a file, or append this contents to existing /etc/sysctl.conf file. Anyway, at the program's current level (no .dmg installation file), adventurous Mac users will be able to do this change.