Add security policy and update markdown docs, bump jest version

[asears]

Description

• Add a security policy to clarify how to address security issues with the extension.
• Clarify that public gists will show up after install and before assigning a PAT token.
• Bump jest version after local setup/tests.
• Update test steps.

Alternative to Dependabot is Whitesource Renovate. https://docs.renovatebot.com/install-github-app/ which may pick up dependency issues better.

Related Issue

#121

Motivation and Context

Security and doc improvements.

How Has This Been Tested?

Ran the tests in VSCode.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• [x ] New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x ] My code follows the code style of this project.
• My change requires a change to the documentation.
• [x ] I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[coveralls]

Coverage remained the same at 82.774% when pulling c13d89a on asears:feature/securitypolicy into 5cb77d5 on kenhowardpdx:develop.

[kenhowardpdx]

I like these changes. Is there a security policy that fits my maintainer style of only checking GitHub about once a month? I really don't want to turn on email notifications. Also, I don't want to be held accountable for users publishing things they don't want to world to see. Can we make the security policy can point to GitHub's security policy and be a pass-through to theirs? 🤷🏼‍♂️

[kenhowardpdx]

I don't want to be beholden to 48 hours. I like the concept of a security policy... but not sure I will actually be able to adhere to this.

[asears]

I think you can merge the PR and do as you wish? These are the defaults, I'm sure you can tailor them as you like for a personal github project.

https://snyk.io/blog/ten-git-hub-security-best-practices/

Glad I could contribute and appreciate your speedy response to the initial security questions I had and appreciate your work on this extension.