PR #17522: Update security section.

Imported from GitHub PR #17522

Added a section to address the security practices and to report security vulnerability found in keras.  Guiding users to use the same steps followed for Tensorflow repo.
Fixes: https://github.com/keras-team/keras/issues/17459
Copybara import of the project:

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3839676 by Sachin Prasad <sachinprasad@google.com>:

Update security section.

Added a section to address the security practices and to report security vulnerability found in keras.
Guiding users to use the same steps followed for Tensorflow repo.
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433f7da by Sachin Prasad <sachinprasad@google.com>:

Update Readme file
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d16d9b3 by Sachin Prasad <sachinprasad@google.com>:

Add security policy details
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8beb4c2 by Sachin Prasad <sachinprasad@google.com>:

Add security policy details

Merging this change closes #17522

FUTURE_COPYBARA_INTEGRATE_REVIEW=#17522 from keras-team:sachinprasadhs-patch-3 8beb4c2
PiperOrigin-RevId: 512105115

CONTRIBUTING.md

@@ -309,3 +309,8 @@ mind.
     submitting PRs to fix one typo, one warning,etc. We recommend fixing the
     same issue at the file level at least (e.g.: fix all typos in a file, fix
     all compiler warnings in a file, etc.)
+
+## Security vulnerability reports
+
+Since Keras is the high-level API of TensorFlow 2, Keras follows same security practices as TensorFlow.
+For details on guidelines on vulnerabilities and reporting them, you can refer [Using TensorFlow Securely](https://github.com/tensorflow/tensorflow/blob/master/SECURITY.md).