Create a security issue template

[Kouteki]

Current state

As a researcher/user who found a security issue, I have to choose between opening a public issue or looking for more private channels, risking to contact the wrong person in the process.

Desired state

As a researcher/user who found a security issue, I can click New issue and get instructions how to properly report it to the gno.land team.

Acceptance criteria

• [ ] Add the security report as a New issue option
• [ ] Give sufficient information on the triage process
• [ ] Give instructions how to submit a report
• [ ] Ensure that all the stakeholders receive the report

[Kouteki]

I googled some best practices from github :)

https://docs.github.com/en/code-security/getting-started/adding-a-security-policy-to-your-repository