Added security considerations document (#3488)

* Added security considerations document

* Update SECURITY_CONSIDERATIONS.md

Co-authored-by: Asbjørn Ulsberg <asbjorn@ulsberg.no>

* Update SECURITY_CONSIDERATIONS.md

Co-authored-by: Asbjørn Ulsberg <asbjorn@ulsberg.no>

* Add note about external references being potentially on untrusted domains

* Update SECURITY_CONSIDERATIONS.md

Co-authored-by: Ralf Handl <ralf.handl@sap.com>

* Update SECURITY_CONSIDERATIONS.md

Co-authored-by: Lorna Jane Mitchell <lorna@lornajane.net>

* Added note about security schemes

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Co-authored-by: Asbjørn Ulsberg <asbjorn@ulsberg.no>
Co-authored-by: Ralf Handl <ralf.handl@sap.com>
Co-authored-by: Lorna Jane Mitchell <lorna@lornajane.net>