Link the documentation in README.md and add details.

README.md

@@ -1,2 +1,2 @@
 # data-example
-Example dataset used in documentation
+Example dataset used in ADS [documentation](https://axondeepseg.readthedocs.io/en/latest/documentation.html#example-dataset). This example dataset contains **2 SEM** test samples and **1 TEM** test sample.