Update README for 3.1.0

Author: isabelatkinson

README.md

@@ -39,7 +39,7 @@ import PackageDescription
 let package = Package(
     name: "MyPackage",
     dependencies: [
-        .package(url: "https://github.com/mongodb/swift-bson", .upToNextMajor(from: "3.0.2"))
+        .package(url: "https://github.com/mongodb/swift-bson", .upToNextMajor(from: "3.1.0"))
     ],
     targets: [
         .target(name: "MyTarget", dependencies: ["SwiftBSON"])