Docs: note about pip editable mode.

docs/contributing.rst

@@ -17,6 +17,15 @@ Now you can install the development packages:
 
    ./venv/bin/pip install -r requirements.txt -e .[dev,docs,testing]
 
+.. note:: `-e` (short for `--editable`) turns on *editable mode* in
+          which any source code changes in the current working
+          directory are "live" and no further `pip install ...`
+          invocations are necessary while developing. Any `pip install
+          .` call will re-install Let's Encrypt in non-editable mode.
+
+          This is roughly equivalent to `python setup.py develop`. For
+          more info see `man pip`.
+
 The code base, including your pull requests, **must** have 100% test
 statement coverage **and** be compliant with the :ref:`coding style
 <coding-style>`.