chore: Add security issue notification to readme

[seebees]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Check any applicable:

• Were any files moved? Moving files changes their URL, which breaks all hyperlinks to the files.

[lizroth]

Now that the checks are running again, this caught an actual issue

(line 42) broken https://github.com/aws/aws-dynamodb-encryption-python/tree/master/CONTRIBUTING.md#security-issue-notifications - Anchor 'security-issue-notifications' not found

Looking into options to fix this up in this PR if possible

Edit: I take it back. I thought there must be a typo or something, but the anchor does appear to exist/work. False negative?

[lizroth]

The #security-issue-notifications anchor works, but the actual a id= value is <a id="user-content-security-issue-notifications". Sphinx's linkcheck is looking at the id value directly, I suppose, so the check was showing up as broken. Using the real/raw ID value seems to do the trick in my local testing.