Added an API key token provider.

README.md

@@ -46,11 +46,17 @@ let client = Client(
 )
 ```
 
-Tokens expire after 20 minutes at most. For longer-lived clients, pass a token provider instead, which is called on every request:
+Tokens expire after 20 minutes at most. For longer-lived clients, use `APIKeyTokenProvider`, which signs new ones with your API key as needed:
 
 ```swift
+let tokenProvider = try APIKeyTokenProvider(
+    issuerID: yourIssuerID,
+    keyID: yourKeyID,
+    privateKey: yourP8FileContents
+)
+
 BearerAuthMiddleware {
-    try await tokenStore.validToken()
+    try await tokenProvider.token()
 }
 ```
 
@@ -82,11 +88,11 @@ repeat {
 
 ## Supported Platforms
 
-- iOS 13.0+
-- macOS 10.15+
-- tvOS 13.0+
+- iOS 14.0+
+- macOS 11.0+
+- tvOS 14.0+
 - visionOS 1.0+
-- watchOS 6.0+
+- watchOS 7.0+
 
 ## Development