fix: update logo in README with responsive images (#80)

Author: Ryan-Amirthan

README.md

@@ -1,7 +1,11 @@
 <br/>
 <div align="center">
-  <a href="https://www.buildwithfern.com/?utm_source=github&utm_medium=readme&utm_campaign=docs-starter-openapi&utm_content=logo">
-    <img src="/fern/docs/assets/fern-logo.png" height="50" align="center" alt="header" />
+  <a href="https://www.buildwithfern.com/?utm_source=github&utm_medium=readme&utm_campaign=fern&utm_content=logo">
+    <picture>
+      <source media="(prefers-color-scheme: dark)" srcset="/fern/docs/assets/fern-logo-white.svg">
+      <source media="(prefers-color-scheme: light)" srcset="/fern/docs/assets/fern-logo-primary.svg">
+      <img alt="logo" src="/fern/docs/assets/fern-logo-primary.svg" height="50" align="center">
+    </picture>
   </a>
 
   <br/>