Updated README.md to include PYTHONPATH inclusion

Since pipenv only installs dependencies, but not the streamflow module
itself, it is necessary to add the streamflow project folder to the
PYTHONPATH environment variable in order to make it work. This is
done automatically by all IDEs, but in case of manual execution it
is still necessary

.travis.yml

@@ -17,7 +17,7 @@ before_deploy:
       git config --local user.name "alpha-unito-bot"
       git config --local user.email "[email protected]"
       export TRAVIS_TAG="${STREAMFLOW_VERSION}"
-      git tag ${TRAVIS_TAG}
+      git tag -a ${TRAVIS_TAG} -m "StreamFlow Release ${STREAMFLOW_VERSION}"
       # Prepare pipenv
       pipenv lock -r > ${TRAVIS_BUILD_DIR}/requirements.txt
     fi

README.md

@@ -89,7 +89,9 @@ cd streamflow
 pipenv install
 ```
 
-Finally, you can run StreamFlow in the generated virtual environment
+Finally, you can run StreamFlow in the generated virtual environment. In order for this to work, it is
+necessary to add the streamflow project folder (the one generated by the `git clone` command) to your
+`PYTHONPATH` list
 ```bash
 pipenv run python -m streamflow
 ```