document/fix missing IPC_LOCK capability

If cap_ipc_lock capability is not available, it currently fails with something like:


```shell
~$ vac                     
ERRO[0000] error locking vac memory: cannot allocate memory
```

Workaround for now is to give it the capability, whether at the user level or on the binary, eg:

```shell
~# setcap 'cap_ipc_lock+ep' /home/linuxbrew/.linuxbrew/Cellar/vac/0.0.6/bin/vac
```
