Add precisions about the creation of the socket directory

[hug-dev]

Reported by @anta5010 on Slack here

The problem is: on most (all?) modern linuxes “/run” is tpmfs and is not persistent during reboots and “/run/parsec” needs to be created again after each reboot.

We should document that in the installation section of the book and give some solutions.

Fedora uses the tmpfiles.d service to automatically create the /run directories.

For example:

[hugdev@machine]$ cat /usr/lib/tmpfiles.d/parsec.conf
#Type   Path          Mode User   Group            Age         Argument
d       /run/parsec   750  parsec parsec-clients   -           -