Add security.md (#201)

SECURITY.md

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+# Security
+
+If you believe you have found a security vulnerability in any SecretFlow repository that meets
+[SecretFlow's definition of a security vulnerability](https://security.alipay.com/announcement.htm?id=1), please report it to us as described below.
+
+## Reporting Security Issues
+
+**Please do not report security vulnerabilities through public GitHub issues.**
+
+Instead, please report them to the ANT GROUP SECURITY Response Center at [https://security.alipay.com/](https://security.alipay.com/).
+
+If you prefer to submit without logging in, send email to [[email protected]](mailto:[email protected]).
+
+You should receive a response within 48 hours. If for some reason you do not, please follow up via email to ensure we received your original message.
+Additional information can be found at [https://security.alipay.com/](https://security.alipay.com/).
+
+Please include the requested information listed below (as much as you can provide) to help us better understand the nature and scope of the possible issue:
+
+* Type of issue (e.g. buffer overflow, SQL injection, cross-site scripting, etc.)
+* Full paths of source file(s) related to the manifestation of the issue
+* The location of the affected source code (tag/branch/commit or direct URL)
+* Any special configuration required to reproduce the issue
+* Step-by-step instructions to reproduce the issue
+* Proof-of-concept or exploit code (if possible)
+* Impact of the issue, including how an attacker might exploit the issue
+
+This information will help us triage your report more quickly.
+
+## Preferred Languages
+
+We prefer all communications to be in Chinese or English.