plugins/slurm: slurmadmcli add some documentation

# LIBSLURMADMCLI

libslurmadmcli is a Slurm SPANK¹ plugin handling the ADMIRE adhoc

storage CLI. It is a simple interface between Slurm and the Scord

storage coordination service².

# INSTALLATION

See ../Makefile for details on how to compile libslurmadmcli.so. Note

that it only depends on the Slurm and the Scord C libraries, and

access to the slurm/spank.h header.

Once the shared object is compiled, it only needs to be moved

somewhere the Slurm plugin system can pick it up. For example, if

libslurmadmcli.so is installed in /usr/local/lib/, the following line

can be added to the Slurm plugin configuration file (usually

/etc/slurm/plugstack.conf)

	optional	/usr/local/lib/libslurmadmcli.so	scord_addr=ofi+tcp://127.0.0.1:52000 scord_proto=ofi+tcp://127.0.0.1 scordctl_bin=scord-ctl

The key-value pairs following the plugin are optional configuration

variables.

- scord_addr: The address to contact the Scord service in Mercury format

  (default to ofi+tcp://127.0.0.1:52000)

- scord_proto: The transport protocol to use in Mercury format (default

  to ofi+tcp).

- scordctl_bin: The scord-ctl binary to run on every node of an

  allocation, can be the path to an executable (default to scord-ctl).

# REFERENCES

1. See manual page spank(7) and <slurm/spank.h>

2. See https://storage.bsc.es/gitlab/eu/admire/io-scheduler