changelog: add #592

.changelog/unreleased/features/592-implicit-vp.md

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+- Added a validity predicate for implicit accounts. This is set in
+  protocol parameters and may be changed via governance. Additionally,
+  added automatic public key reveal in the client that use an implicit
+  account that hasn't revealed its PK yet as a source. It's also
+  possible to manually submit reveal transaction with  client command
+  ([#592](https://github.com/anoma/namada/pull/592))