Fix for Pystac ItemCollections (#69)

Author: scottyhq

README.md

@@ -8,10 +8,12 @@ For more information and examples, please [see the documentation](https://stacks
 
 ```python
 import stackstac
-import satsearch
+import pystac_client
 
-stac_items = satsearch.Search(
-    url="https://earth-search.aws.element84.com/v0",
+URL = "https://earth-search.aws.element84.com/v0"
+catalog = pystac_client.Client.open(URL)
+
+stac_items = catalog.Search(
     intersects=dict(type="Point", coordinates=[-105.78, 35.79]),
     collections=["sentinel-s2-l2a-cogs"],
     datetime="2020-04-01/2020-05-01"