Update podspec version and added description to changelog (#763)

Author: mdmathias

AppAuth.podspec

@@ -1,7 +1,7 @@
 Pod::Spec.new do |s|
 
   s.name         = "AppAuth"
-  s.version      = "1.6.0"
+  s.version      = "1.6.1"
   s.summary      = "AppAuth for iOS and macOS is a client SDK for communicating with OAuth 2.0 and OpenID Connect providers."
 
   s.description  = <<-DESC

CHANGELOG.md

@@ -1,3 +1,6 @@
+# 1.6.1
+- Increased minimum iOS and macOS versions to fix [build issue](https://github.com/openid/AppAuth-iOS/pull/761)
+
 # 1.6.0
 - Added a `prefersEphemeralSession` parameter for external user-agents. ([#645](https://github.com/openid/AppAuth-iOS/pull/645))
 - Fixed errors encountered when using secure coding to decode `OIDAuthState`. ([#656](https://github.com/openid/AppAuth-iOS/pull/656), [#721](https://github.com/openid/AppAuth-iOS/pull/721))