Create SECURITY.md

[GustavLero]

Description of work

Add your own description here

To test

Which ticket does this PR fix?

Acceptance criteria

List the acceptance criteria for the PR

---

Code Review

• Is the code of an acceptable quality?
• Has the author taken into account the multi-threaded nature of the code?
• Have the changes been documented in the release notes. If so, do they describe the changes appropriately?
• Has the manual system tests spreadsheet been updated?

Functional Tests

• Do changes function as described? Add comments below that describe the tests performed.

Final steps

• Reviewer has updated the submodule in the main EPICS repo? See Reviewing work for the subModules of EPICS in the Git workflow page for details.
• Reviewer has moved the release notes entry for this ticket in the "Changes merged into master" section