Fix RELEASE.md included in package build

Fixes the following warning when using this package as a dependency in another swift project, specifically when running `swift build`

warning: 'swift-prometheus': found 1 file(s) which are unhandled; explicitly declare them as resources or exclude from the target

Author: vimscientist69

Package.swift

@@ -34,6 +34,9 @@ let package = Package(
             dependencies: [
                 .product(name: "Atomics", package: "swift-atomics"),
                 .product(name: "CoreMetrics", package: "swift-metrics"),
+            ],
+            exclude: [
+                "RELEASE.md",
             ]
         ),
         .testTarget(