RFR: 8340815: Add SECURITY.md file [v3]

[George Adams]

> Currently the [security tab](https://github.com/openjdk/jdk/security) on the GitHub repos is empty with no clear information or links on where to report security vulnerabilities.
> 
> <img width="1278" alt="Screenshot 2024-09-24 at 14 28 37" src="https://github.com/user-attachments/assets/4fd68f9f-46d8-4c06-ad71-52747c8f5cf2">
> 
> I've added a simple SECURITY.md file which includes the link to the official policy on the website.

George Adams has updated the pull request incrementally with one additional commit since the last revision:

  Feedback from Mark

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/21155/files
  - new: https://git.openjdk.org/jdk/pull/21155/files/ae4be8e8..9541c78d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21155&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21155&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21155.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21155/head:pull/21155

PR: https://git.openjdk.org/jdk/pull/21155