Docs: Add advanced workflows for massive libraries (Multi-threading & Fast Scan)

[iNdioNicarao]

This PR adds an Advanced Workflows section to the README for users managing large installer libraries.

Running deep .bin extraction sequentially on a massive collection can take several days. This documentation outlines two methods to optimize the process:

1. Quick Security Audits: Documents the -s -B flag combination. This allows users who only need to verify digital signatures for malware/tampering (without checking for bit-rot) to instantly scan their library without extracting data.
2. Multi-Threaded Scanning: Provides an xargs -P example to run gogcheck concurrently. For users with multi-core CPUs and fast NVMe SSDs, this allows running multiple deep extractions simultaneously, significantly reducing the total verification time.

[iNdioNicarao]

Just wanted to provide some context: I recently used gogcheck to verify a massive library of over 1,300 installers. Running it sequentially with full data extraction would have taken several days, but utilizing these two strategies allowed me to secure and verify the entire collection in a fraction of the time.

I figured these workflows would be super helpful to add to the README for anyone else dealing with huge archival drives. Thanks for building such an awesome tool!