Add Migration md file (#11335)

Author: sjrl

MIGRATION.md

@@ -0,0 +1,72 @@
+# Migration Guide
+
+This document is meant to provide a guide for migrating from Haystack v2.X to v3.0.
+
+---
+
+## How to Document a Breaking Change
+
+When you merge a breaking change into the v3 branch, add an entry to this file under the appropriate section below.
+Follow this structure:
+
+### Entry template
+
+```markdown
+### <Short title describing what changed>
+
+**What changed:** One or two sentences describing the change — what was removed, renamed, or altered.
+
+**Why:** Brief motivation (e.g. simplification, API consistency, dependency reduction).
+
+**How to migrate:**
+
+Before (v2.x):
+\`\`\`python
+# example using the old API
+from haystack.components.foo import OldComponent
+component = OldComponent(old_param="value")
+\`\`\`
+
+After (v3.0):
+\`\`\`python
+# example using the new API
+from haystack.components.foo import NewComponent
+component = NewComponent(new_param="value")
+\`\`\`
+```
+
+### Tips
+
+- **One entry per breaking change.** Don't bundle unrelated changes into a single entry.
+- **Include a working code example** for every rename, removal, or signature change.
+- **Link to the PR** when extra context would help (e.g. `See [#1234](https://github.com/deepset-ai/haystack/pull/1234)`).
+
+---
+
+## Breaking Changes
+
+<!-- Add entries here as v3 development progresses. Example below shows the expected format. -->
+
+### Example entry: `Document.dataframe` field removed
+
+**What changed:** The `dataframe` field on `Document` and the `ExtractedTableAnswer` dataclass have been removed. `pandas` is no longer a required dependency.
+
+**Why:** Reduces the default installation footprint. Components that need `pandas` will raise an informative error prompting the user to install it explicitly.
+
+**How to migrate:**
+
+Before (v2.x):
+```python
+from haystack.dataclasses import Document
+import pandas as pd
+
+doc = Document(content=pd.DataFrame({"col": [1, 2, 3]}))
+```
+
+After (v3.0):
+```python
+# Store tabular data as plain content or create a custom component that returns pandas DataFrames as needed.
+from haystack.dataclasses import Document
+
+doc = Document(content="col\n1\n2\n3")
+```