README.md: Added requirements of the expected output.

README.md

 # Tree Edit Distance similarity join - datasets scripts
 
+This repository contains all resources to download and process the datasets for
+tree similarity join experiments.
+
+**We do not store the datasets**, only the scripts to obtain and prepare them.
+
 ## Datasets description
 
 Currently we support the following datasest:
 - **bozen** - Bozen streets
 - **dblp** - DBLP
 
+The details about each dataset can be found in the corresponding README files.
+
 ## Repository organisation
 
 Each dataset and its corresponding scripts belong to a separate directory with a name identifying the dataset.
+
+## Expected output
+
+Each output dataset must satisfy the following requirements:
+- The output dataset must be a single text file with one tree per line.
+- The trees must be in bracket notation.
+- The trees must be sorted by size.