Added descrition on how to use virtual environment (#309)

.gitignore

@@ -49,3 +49,6 @@ coverage.xml
 docs/_build/
 
 tests/welt2000/data/WELT2000.TXT
+
+# Virtual Environment for python
+.env

README.rst

@@ -30,14 +30,22 @@ Development Environment
 -----------------------
 
 If you want to work on aerofiles you should install the necessary dependencies
-using::
+using pip, preferably in a virtual environment::
 
+    $ python -m venv .env
+    $ source .env/bin/activate
     $ pip install -r requirements-dev.txt
 
 You can run the testsuite with::
 
     $ make test
 
+If you close the current shell, the virtual python environment will
+also be deactivated. To continue working in the environment and using
+the installed requirements, you must use this again::
+  
+    $ source .env/bin/activate
+
 Building Docs
 -------------