README.md: dark/light mode logo

Display README logo optimized for dark/light backgrounds.
This is particularly helpful for transparent PNG images.

Signed-off-by: andros21 <[email protected]>

Author: andros21

Diff for: README.md

@@ -1,4 +1,8 @@
-![Firecracker Logo Title](docs/images/fc_logo_full_transparent-bg.png)
+<picture>
+   <source media="(prefers-color-scheme: dark)" srcset="docs/images/fc_logo_full_transparent-bg_white-fg.png">
+   <source media="(prefers-color-scheme: light)" srcset="docs/images/fc_logo_full_transparent-bg.png">
+   <img alt="Firecracker Logo Title" width="750" src="docs/images/fc_logo_full_transparent-bg.png">
+</picture>
 
 Our mission is to enable secure, multi-tenant, minimal-overhead execution of
 container and function workloads.