Improve Security Policy score

[julien-deramond]

Description

Our current score for the Security Policy is apparently 9/10.

To get the last one:

Security Policy Specific Text (1/10 points):

- Specific text providing basic or general information about vulnerability and disclosure practices, expectations, and/or timelines
- Text should include a total of 2 or more hits which match (case-insensitive) 
  - `vuln` and as in "Vulnerability" or "vulnerabilities";
  - `disclos` as "Disclosure" or "disclose";
  - and numbers which convey expectations of times, e.g., 30 days or 90 days

I reused the content of our Incident Response Plan, and linked it.