add release notes for v20

release_notes/changelog-20.md

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+What's New in 20
+================
+
+Release 20 is a **consensus-breaking release** that introduces the first version
+of the Stacks blockchain.  Versions 19 and prior are *incompatible* with this
+release and will diverge from v20 nodes.
+
+Release Highlights
+------------------
+
+**Native Stacks Token**.  This release completes the first version of the Stacks
+blockchain.  Details can be found on this [forum
+post](https://forum.blockstack.org/t/blockstack-annual-hard-fork-2018/6518).
+
+**Blockstack Accounts API**.  This release adds `/v1/accounts` endpoints to the
+RESTful API for querying account balances and transaction histories.  Please see
+the relevant [documentation](https://core.blockstack.org) for details.
+
+Getting Started
+---------------
+
+The recommended way to get started with v20 is through its fast-sync mode.  To
+migrate from a v19 node to a v20 node, we recommend the following steps:
+
+```
+# stop the old node
+$ blockstack-core stop
+$ mv ~/.blockstack-server ~/.blockstack-server.19
+
+# start the new node
+$ blockstack-core fast-sync
+$ blockstack-core start
+```
+
+The old chain state database directory structure has changed, and is 
+incompatible with this new release.  If you are
+upgrading from a v19 node or older, you will need to remove your `.blockstack-server`
+directory.