adds a new section pointing to a relevant doc in Governance that defines how we notify end-users of security issues.

cc @electron/wg-security

Description of Change

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• tests are changed or added
• relevant documentation is changed or added
• PR title follows semantic commit guidelines
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.
• This is NOT A BREAKING CHANGE. Breaking changes may not be merged to master until 11-x-y is branched.

Release Notes

Notes: none