Update SECURITY.md

[spavlusieva]

Thanks for contributing!

Before submitting PRs for python-oracledb you must have your signed Oracle
Contributor Agreement accepted. See https://oca.opensource.oracle.com

If the problem solved is small, you may find it easier to open an Issue
describing the problem and its cause so we can create the fix.

The bottom of your commit message must have the following line using your name
and e-mail address as it appears in the OCA Signatories list.

Signed-off-by: Your Name <you@example.org>

This can be automatically added to pull requests by committing with:

git commit --signoff

[cjbj]

I've pushed to change to our internal repo.