docs: add info that license scanning supports file-patterns flag

[DmitriyLewen]

Description

License scanning with license analyzer supports file-patterns flag:

➜ ls -hl .
-rw-r--r--@ 1 work  staff    32K Nov  1 10:25 my-license-file

➜ trivy fs --scanners license --license-full .                 
2023-11-01T10:25:35.698+0600	INFO	Full license scanning is enabled

➜ trivy fs --scanners license --license-full --file-patterns "license-file:.*my-license-file" .
2023-11-01T10:27:02.501+0600	INFO	Full license scanning is enabled

Loose File License(s) (license)

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

┌────────────────┬──────────┬─────────┬─────────────────┐
│ Classification │ Severity │ License │  File Location  │
├────────────────┼──────────┼─────────┼─────────────────┤
│ Restricted     │ HIGH     │ GPL-3.0 │ my-license-file │
└────────────────┴──────────┴─────────┴─────────────────┘

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).