Add podspec

WeakMapTable.podspec

@@ -0,0 +1,18 @@
+Pod::Spec.new do |s|
+  s.name             = "WeakMapTable"
+  s.version          = "1.0.0"
+  s.summary          = "A weak-to-strong map table"
+  s.homepage         = "https://github.com/ReactorKit/WeakMapTable"
+  s.license          = { :type => "MIT", :file => "LICENSE" }
+  s.author           = { "Suyeol Jeon" => "[email protected]" }
+  s.source           = { :git => "https://github.com/ReactorKit/WeakMapTable.git",
+                         :tag => s.version.to_s }
+  s.source_files = "Sources/**/*.{swift,h,m}"
+  s.frameworks   = "Foundation"
+  s.swift_version = "5.1"
+
+  s.ios.deployment_target = "8.0"
+  s.osx.deployment_target = "10.11"
+  s.tvos.deployment_target = "9.0"
+  s.watchos.deployment_target = "3.0"
+end