Add a Proguard rule for the missing serialization classes

This permits using kotlinx-datetime without kotlinx-serialization
on Android without complaints from the build tooling.

Fixes #297

core/jvm/resources/META-INF/proguard/datetime.pro

@@ -0,0 +1,11 @@
+# We depend on kotlinx-serialization as compileOnly.
+# If the serializers don't get used, the library works properly even without the
+# kotlinx-serialization dependency, but Proguard emits warnings about datetime
+# classes mentioning some serialization-related entities.
+# These rules should not cause problems: if a project actually relies on
+# serialization, then much more than just these two classes will be required,
+# so telling Proguard not to worry if these two are missing will not prevent it
+# from emitting errors for code that does use serialization but somehow forgot
+# to depend on it.
+-dontwarn kotlinx.serialization.KSerializer
+-dontwarn kotlinx.serialization.Serializable