docs: add CNCF code of conduct in readme to meet CNCF's request (sealerio#1525)

Signed-off-by: Allen Sun <shlallen1990@gmail.com>

Author: allencloud

README.md

@@ -125,6 +125,10 @@ sealer run kubernetes:v1.19.8 --masters 192.168.0.2 --passwd xxx
 </div>
 <!-- markdownlint-restore -->
 
+## Code of Conduct
+
+sealer follows the [CNCF Code of Conduct](https://github.com/cncf/foundation/blob/master/code-of-conduct.md).
+
 ## License
 
 Sealer is licensed under the Apache License, Version 2.0. See [LICENSE](LICENSE) for the full license text.

code-of-conduct.md

@@ -1,3 +1,3 @@
-# Sealer Community Code of Conduct
+# sealer Community Code of Conduct
 
-Sealer follows the [CNCF Code of Conduct](https://github.com/cncf/foundation/blob/master/code-of-conduct.md).
+sealer follows the [CNCF Code of Conduct](https://github.com/cncf/foundation/blob/master/code-of-conduct.md).