README: Add BepInExLibs package source

Added BepInExLibs package source to configuration.

Author: ManlyMarco

README.md

@@ -13,6 +13,7 @@ Every library ever needed for Illusion game modding, almost.
     </packageRestore>
     <packageSources>
         <add key="IllusionMods" value="https://pkgs.dev.azure.com/IllusionMods/Nuget/_packaging/IllusionMods/nuget/v3/index.json" />
+        <add key="BepInExLibs" value="https://nuget.bepinex.dev/v3/index.json" />
     </packageSources>
 </configuration>
 ```