Specify where databases are automatically downloaded

Refs #812

README.md

@@ -52,7 +52,7 @@ Every time you will try to commit, pre-commit will run checks on your files to m
 
 ## Usage
 
-Run the `run.py` script to perform training / classification. The first time `run.py` is executed, the `--train` argument should be used to automatically download databases containing bugs and commits data.
+Run the `run.py` script to perform training / classification. The first time `run.py` is executed, the `--train` argument should be used to automatically download databases containing bugs and commits data (they will be downloaded in the data/ directory).
 
 ### Running the repository mining script