add small test datasets

Author: LuisHeinzlmeier

README.md

@@ -37,3 +37,6 @@ git fetch
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 [^1]: From [stackoverflow](https://stackoverflow.com/a/60846265/11502856)
+
+## Datasets 
+- `spot`: Pseudocount test datasets with 900 x 3540 × 22914 (spots x cells × genes)