docs: add SECURITY.md with security policy and best practices

[njg7194]

Summary

This PR adds a SECURITY.md file to establish security reporting guidelines and best practices for users.

Changes

• Added comprehensive SECURITY.md with:
  • Vulnerability reporting process
  • Response timeline expectations
  • Security best practices for key management
  • Algorithm selection recommendations
  • Input validation guidelines
  • Known security considerations

Motivation

As a widely-used cryptography library (3M+ weekly npm downloads), jsrsasign benefits from:

1. Clear vulnerability reporting channels
2. Documented security best practices for users
3. Guidance on secure implementation patterns

This follows GitHub's security best practices and helps protect the community of users who depend on this library.

[kjur]

Hi @njg7194 , thank you kindly for your PR. I've merged it. Thank you.