update migration docs

docs/v3_migration.md

@@ -2,6 +2,110 @@
 
 ### MosaicJSON pydantic model
 
+We now use [pydantic](https://pydantic-docs.helpmanual.io) to define the MosaicJSON document. Pydantic 
+
+From Pydantic docs:
+> Define how data should be in pure, canonical python; validate it with pydantic.
+
+Pydantic model enforce the mosaicjson specification for the whole project by validating each items.
+
+```python
+from pydantic import BaseModel
+
+class MosaicJSON(BaseModel):
+    """
+    MosaicJSON model.
+
+    Based on https://github.com/developmentseed/mosaicjson-spec
+
+    """
+
+    mosaicjson: str
+    name: Optional[str]
+    description: Optional[str]
+    version: str = "1.0.0"
+    attribution: Optional[str]
+    minzoom: int = Field(0, ge=0, le=30)
+    maxzoom: int = Field(30, ge=0, le=30)
+    quadkey_zoom: Optional[int]
+    bounds: List[float] = Field([-180, -90, 180, 90])
+    center: Optional[Tuple[float, float, int]]
+    tiles: Dict[str, List[str]]
+```
+
+##### Validation
+
+```python
+mosaic_definition = dict(
+    mosaicjson="0.0.2",
+    minzoom=1,
+    maxzoom=2,
+    quadkey_zoom=1,
+    bounds=[-180, -90, 180, 90],
+    center=(0, 0, 1),
+    tiles={},
+)
+
+m = MosaicJSON(**mosaic_definition)
+> MosaicJSON(mosaicjson='0.0.2', name=None, description=None, version='1.0.0', attribution=None, minzoom=1, maxzoom=2, quadkey_zoom=1, bounds=[-180.0, -90.0, 180.0, 90.0], center=(0.0, 0.0, 1), tiles={})
+```
+
+```python
+# convert the mode to a dict
+m.dict(exclude_none=True)
+> {'mosaicjson': '0.0.2',
+ 'version': '1.0.0',
+ 'minzoom': 1,
+ 'maxzoom': 2,
+ 'quadkey_zoom': 1,
+ 'bounds': [-180.0, -90.0, 180.0, 90.0],
+ 'center': (0.0, 0.0, 1),
+ 'tiles': {}}
+```
+
+```python
+mosaic_definition = dict(
+    mosaicjson="0.0.2",
+    minzoom=1,
+    maxzoom=100,
+    quadkey_zoom=1,
+    bounds=[-180, -90, 180, 90],
+    center=(0, 0, 1),
+    tiles={},
+)
+
+m = MosaicJSON(**mosaic_definition)
+...
+ValidationError: 1 validation error for MosaicJSON
+maxzoom
+  ensure this value is less than or equal to 30 (type=value_error.number.not_le; limit_value=30)
+```
+
+### Creation
+
+The `MosaicJSON` class comes also with helper functions: 
+- **MosaicJSON.from_urls**: Create a mosaicjson from a set of COG urls
+- **MosaicJSON.from_features**: Create a mosaicjson from a set of GeoJSON features
+- **MosaicJSON._create_mosaic** (semi-private): Low level mosaic creation methods used by public methods (`from_urls` and `from_features`).
+
+```python
+#V2
+from cogeo_mosaic.utils import create_mosaic
+
+mosaic_definition: Dict = create_mosaic(dataset)
+
+
+#V3
+from cogeo_mosaic.mosaic import MosaicJSON
+
+mosaic_definition: MosaicJSON = MosaicJSON.from_urls(dataset)
+
+# or from a list of GeoJSON Features
+mosaic_definition: MosaicJSON = MosaicJSON.from_features(dataset, minzoom=1, maxzoom=3)
+```
+
+
+To learn more about the low-level api checkout [/docs/AdvancedTopics.md](/docs/AdvancedTopics.md)
 
 ### Backend Storage