RFR: 8340954: Add SECURITY.md file

[Kevin Rushforth]

A `SECURITY.md` file was recently added to the jdk repo. GitHub will show that policy if you click on the ["Security" tab](https://github.com/openjdk/jdk/security) of the jdk repo -- If you are logged in, you may need to further click on the ["Policy" tab](https://github.com/openjdk/jdk/security/policy).

We need a copy of this file in the jfx repo, so that similarly, you will see the policy if you click on the ["Security" tab](https://github.com/openjdk/jfx/security) of the jfx repo -- if you are logged in, you may need to further click on the ["Policy" tab](https://github.com/openjdk/jfx/security/policy).

The `SECURITY.md` file in this PR is identical to the one in the jdk repo, with "JDK" replaced by "JavaFX" in two places (the section header and the name of the software).

See openjdk/jdk#21155 for more details.

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Commit messages:
 - 8340954:  Add SECURITY.md file

Changes: https://git.openjdk.org/jfx/pull/1578/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1578&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340954
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1578.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1578/head:pull/1578

PR: https://git.openjdk.org/jfx/pull/1578