docs: add a logo picture for github dark theme

Author: xuruidong

README.md

@@ -1,5 +1,9 @@
 
-<img src="https://github.com/owasp-modsecurity/ModSecurity/raw/v3/master/others/modsec.png" width="50%">
+<picture>
+  <source media="(prefers-color-scheme: dark)" srcset="./others/modsec_white_bg.png">
+  <source media="(prefers-color-scheme: light)" srcset="./others/modsec.png">
+  <img src="./others/modsec.png" width="50%">
+</picture>
 
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