feat: add StellarSplit mark to README header

Author: Kingsman-99

README.md

@@ -1,4 +1,7 @@
-# split-app
+<div align="center">
+  <img src="https://raw.githubusercontent.com/Stellar-split/.github/main/assets/stellarsplit-mark.svg" alt="StellarSplit" width="80" />
+  <h1>split-app</h1>
+</div>
 
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