docs: add 2.0 to 2.1 migration guide

docs/migration-v2.0-v2.1.md

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+# Migration Guide - V2.0 to V2.1
+This version results a very small breaking change in the `datasets.json` file. The `address` field is now parsed as an array/list, so you must enclose it in `[]` brackets. It does not need to contain multiple addresses, one will still work fine.
+
+This change allows one-many mapping between datasets and wallets, as may be the case when ingesting a dataset where the datacap allocation is split across multiple wallets. 
+
+To change this, edit the `datasets.json` file:
+
+
+OLD *~/.delta/importer/datasets.json*
+```jsonc
+[
+  {
+    "dataset": "radiant-ml",
+    "address": "f1p3l3wgnfukemmaupqecwcoqp7fcgjcqgqcq7rja",
+    "dir": "/mnt/delta-datasets/radiant-poc",
+    "ignore": false
+  }
+]
+```
+
+Change to:
+
+NEW *~/.delta/importer/datasets.json*
+```jsonc
+[
+  {
+    "dataset": "radiant-ml",
+    "address": ["f1p3l3wgnfukemmaupqecwcoqp7fcgjcqgqcq7rja"], // <- address is now enclosed in [] brackets
+    "dir": "/mnt/delta-datasets/radiant-poc",
+    "ignore": false
+  }
+]
+```