Add Install Plugin heading and fix heading level in README

[ydesgagn]

Summary

Improve the README installation section by adding a missing heading above the plugin install commands and normalizing the heading hierarchy so sibling sections use the same level.

Key changes:

• Add ### Install Plugin heading above the /plugin marketplace add and /plugin install commands
• Promote #### Recommended Permissions to ### Recommended Permissions for consistent heading hierarchy

Types of changes

• Bugfix (fixes an issue)
• New feature (adds functionality)
• Refactoring (improves code without changing functionality)
• Breaking change (incompatible changes)
• Build or security update (updates dependencies, libraries, or security patches)
• Code style or documentation update (formatting, renaming, or documentation changes)
• Other (please describe):

Checklist

• Unit tests added to validate my fix/feature
• I have manually tested my change
• I did not add automation test. Why ?:
• Database changes requiring migration with downtime or reprocessing of existing data
• The SOUP file lists the risk Level, requirements and verification reasoning associated with each library
• readme.md includes sections on introduction, installation, usage, and contributing
• docs/architecture.md includes sections on the architecture diagram, software units, software of unknown provenance, critical algorithms and risk controls related to PII and security
• Impact on PII, privacy regulations (CCPA/GDPR/PIPEDA), CIS benchmarks or security (availability/confidentiality/integrity); management must be notified