Add bootstrapped pip dependency

README.md

@@ -22,6 +22,11 @@ pip install openslide-python
 ```
 or follow the instructions [here](https://openslide.org/download/).
 
+To install `bootstrapped`:
+```bash
+pip install bootstrapped
+```
+
 **Input files**
 
 * omics layer 1 data: samples x features, tab-separated, with row & column names