updated README.md

- signed-off-by: trimstray <trimstray@gmail.com>

Author: trimstray

README.md

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-    <img src="https://github.com/trimstray/the-practical-linux-hardening-guide/blob/master/doc/img/main_preview.jpg"
+    <img src="https://github.com/trimstray/the-practical-linux-hardening-guide/blob/master/static/img/main_preview.jpg"
         alt="Master">
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-<p align="center"><code>I'm back, work in progress...</code>
-
 ****
 
 # Table of Contents
@@ -86,6 +84,16 @@ Please also remember:
 
   > This guide also contains my comments that may differ from certain industry principles. If you are not sure what to do please see **[Policy Compliance](#policy-compliance)**.
 
+This guide use following [OpenSCAP](https://www.open-scap.org/) configurations:
+
+- [U.S. Government Commercial Cloud Services (C2S) baseline inspired by CIS v2.1.1](https://static.open-scap.org/ssg-guides/ssg-rhel7-guide-C2S.html)
+
+  > C2S for Red Hat Enterprise Linux 7 v0.1.43.
+
+- [Red Hat Enterprise Linux 7 Security Technical Implementation Guide (STIG)](https://www.stigviewer.com/stig/red_hat_enterprise_linux_7/)
+
+  > The requirements are derived from the (NIST) 800-53 and related documents.
+
 ### The Importance of Hardening Linux
 
 Simply speaking, hardening is the process of making a system more secure. Out of the box, Linux servers don’t come "hardened" (e.g. with the attack surface minimized). It’s up to you to prepare for each eventuality and set up systems to notify you of any suspicious activity in the future.