Update branding in README.md (ton-blockchain#566)

Light logo from https://ton.org/download/ton_logo_light_background.svg for GitHub Light Theme
and Dark logo from https://ton.org/download/ton_logo_dark_background.svg for GitHub Dark Theme

README.md

@@ -1,3 +1,11 @@
+<div align="center">
+  <picture>
+    <source media="(prefers-color-scheme: dark)" srcset="https://ton.org/download/ton_logo_dark_background.svg">
+    <img alt="TON logo" src="https://ton.org/download/ton_logo_light_background.svg">
+  </picture>
+  <hr/>
+</div>
+
 [![TON Overflow Group][ton-overflow-badge]][ton-overflow-url]
 [![Stack Overflow Group][stack-overflow-badge]][stack-overflow-url]
 [![Telegram Foundation Group][telegram-foundation-badge]][telegram-foundation-url]
@@ -17,8 +25,6 @@
 
 
 
-# TON
-
 Main TON monorepo, which includes the code of the node/validator, lite-client, tonlib, FunC compiler, etc.
 
 ## Updates flow: