added TODO

TODO.md

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+# architecture
+
+* class Resolver, public interface, hold private ref to dnscrypt client
+* class DNSCryptClient, private interface, implements dns api methods
+* receive certificate using something like `_construct`.
+* drop state management using `next-state`, recreate client when certificate expired.
+* class Session used to store session sensitive data, like certificate.
+* implement TCP and UDP transport using different classes, hide impl details there.
+* do not use global default resolver due to it's need explicit state management.
+* leave encrypt / decrypt / verify in a module, just rename it.