docs: add Singularity HPC usage guidance to README

[mtinti]

Motivation

• Add guidance for running the pipeline with Singularity on shared HPC filesystems so users know to provide absolute host paths and bind the project directory when launching Snakemake.

Description

• Added a new Running with Singularity on HPC filesystems section to README.md that includes the requested snakemake command with --use-singularity and --singularity-args, an example config snippet showing absolute processing_dir, results_dir, and benchmark_dir, and a link to the mtinti/myRna-seq-docker repository.

Testing

• Documentation-only change; no automated tests were required or run, and repository sanity actions (commit) completed successfully.

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Codex Task