docs: add notes about mutation (#192)

* docs: add notes about mutation

* docs: try gfm

Author: gadomski

README.md

@@ -41,6 +41,14 @@ import antimeridian
 fixed = antimeridian.fix_geojson(geojson)
 ```
 
+> [!NOTE]
+> `fix_geojson` mutates the input dictionary in-place. If you need to preserve the original GeoJSON, make a copy first:
+>
+> ```python
+> import copy
+> fixed = antimeridian.fix_geojson(copy.deepcopy(geojson))
+> ```
+
 We also have some utilities to create [bounding boxes](https://antimeridian.readthedocs.io/en/latest/api.html#antimeridian.bbox) and [centroids](https://antimeridian.readthedocs.io/en/latest/api.html#antimeridian.centroid) from antimeridian-crossing polygons and multipolygons.
 See [the documentation](https://www.gadom.ski/antimeridian/) for a complete API reference.
 

src/antimeridian/_implementation.py

@@ -85,6 +85,11 @@ def fix_geojson(
     See [antimeridian.fix_polygon][] for a description of the `force_north_pole`
     `force_south_pole` and `fix_winding` arguments.
 
+    Warning:
+        This function mutates the input dictionary in-place. If you need to
+        preserve the original GeoJSON, make a copy before calling this function
+        (e.g., using `copy.deepcopy(geojson)`).
+
     Args:
         geojson: A GeoJSON object as a dictionary
         force_north_pole: If the polygon crosses the antimeridian, force the
@@ -190,6 +195,11 @@ def fix_shape(
     See [antimeridian.fix_polygon][] for a description of the `force_north_pole`
     `force_south_pole` and `fix_winding` arguments.
 
+    Note:
+        When `shape` is a dictionary, this function does not mutate it directly,
+        but converts it to a Shapely geometry first. However, if called via
+        [antimeridian.fix_geojson][], the input dictionary will be mutated.
+
     Args:
         shape: A polygon, multi-polygon, line string, or multi-line string,
             either as a dictionary or as a [antimeridian.GeoInterface][]. Uses