added migration guide for v2

MIGRATING.md

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+# Migrating Guide
+
+## Moving to mustache.js v2
+
+### Overview
+
+mustache.js v2 introduces a bug fix that breaks compatibility with older versions: fixing null and undefined lookup.
+
+When mustache.js tries to render a variable `{{name}}`, it executes a `lookup` function to figure out which value it should render. This function looks up the value for the key `name` in the current context, and if there is no such key in the current context it looks up the parent contexts recursively.
+
+Value lookup should stop whenever the key exists in the context. However, due to a bug, this was not happening when the value was `null` or `undefined` even though the key existed in the context.
+
+Here's a simple example of the same template rendered with both mustache.js v1 and v2:
+
+Template:
+```mustache
+{{#friends}}
+{{name}}'s twitter is: {{twitter}}
+{{/friends}}
+```
+
+View:
+```json
+{
+    "name": "David",
+    "twitter": "@dasilvacontin",
+    "friends": [
+        {
+            "name": "Phillip",
+            "twitter": "@phillipjohnsen"
+        },
+        {
+            "name": "Jan",
+            "twitter": null
+        }
+    ]
+}
+```
+
+Rendered using mustache.js v1:
+```text
+Phillip's twitter is: @phillipjohnsen
+Jan's twitter is: @dasilvacontin
+```
+
+Rendered using mustache.js v2:
+```text
+Phillip's twitter is: @phillipjohnsen
+Jan's twitter is:
+```