docs: add info about support for package license detection in fs/repo modes

[DmitriyLewen]

Description

Trivy supports license detection for some lock files.
e.g.:

➜  trivy -q fs --scanners license ./integration/testdata/fixtures/repo/yarn

yarn.lock (license)

Total: 1 (UNKNOWN: 0, LOW: 1, MEDIUM: 0, HIGH: 0, CRITICAL: 0)

┌─────────┬─────────┬────────────────┬──────────┐
│ Package │ License │ Classification │ Severity │
├─────────┼─────────┼────────────────┼──────────┤
│ jquery  │ MIT     │ Notice         │ LOW      │
└─────────┴─────────┴────────────────┴──────────┘

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).