Update secure_coding_guidelines.md

Greg Alfarorequested to merge
truegreg-master-patch-09923 into master

What does this MR do and why?

This creates guidance on avoiding using local storage for sensitive data. Unsure if we need a tech writer review for this.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

ping @gitlab-com/gl-security/appsec

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