add how-to for getting started on existing project

ref #1937

doc/en/getting-started.rst

@@ -185,6 +185,21 @@ You can find out what kind of builtin :ref:`fixtures` exist by typing::
 
     pytest --fixtures   # shows builtin and custom fixtures
 
+Contributing tests to an existing project
+-------------------------------------
+
+Say you want to contribute some tests to an existing repository.  
+After pulling the code into your development space using some 
+flavor of version control and (optionally) setting up a virtualenv
+you will want to run::
+
+    pip install -e  # Environment dependent alternatives include 
+                    # 'python setup.py develop' and 'conda develop' 
+
+in your project root.  This will set up a symlink to your code in
+site-packages, allowing you to edit your code while your tests
+run against it as if it were installed.
+
 Where to go next
 -------------------------------------