Update UPGRADE.md

Author: andrewosh

UPGRADE.md

@@ -0,0 +1,12 @@
+If you're migrating from `corestore-swarm-networking` v5 to `@corestore/networker` v1, there have been a few breaking changes to be aware of:
+
+### `join`/`leave` -> `configure`
+`join` and `leave` have been replaced by a single method, `configure`, which takes the options `announce`, `lookup`, and `flushed`. 
+
+Using `configure`, a `join` is `configure(discoveryKey, { announce: true, lookup: true }` and a `leave` is `configure(discoveryKey, { announce: false, lookup: false })`
+
+### The `status` method
+The `status` method previously just fowarded Hyperswarm's `status`. It now returns the current network configuration for a given discovery key. To get the Hyperswarm status directly, you can do `networker.swarm.status`.
+
+### Other Assorted Changes
+* The public `streams` and `peers` properties are now `Set`s instead of Arrays.