[MERGE #476] Add a copy of the security notice to CONTRIBUTING.md.
[MERGE #476] Add a copy of the security notice to CONTRIBUTING.md.

Merge pull request #476 from dilijev:contrib-security
This notice was in the README but not in CONTRIBUTING. @digitalinfinity and I think it is better to be redundant on this one as it is so important, and people opening issues or PRs may be more likely to see it if it is in the link pointed to in the banner that pops up when issues and PRs are opened.

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