Update README.rst

Co-authored-by: Chris Markiewicz <[email protected]>

Author: ZviBaratz

README.rst

@@ -104,6 +104,10 @@ To install the latest development version, run::
 
    pip install git+https://github.com/nipy/nibabel
 
+When working on NiBabel itself, it may be useful to install in "editable" mode::
+
+   git clone https://github.com/nipy/nibabel.git
+   pip install -e ./nibabel
 For more information on previous releases, see the `release archive`_ or `development changelog`_.
 
 .. _current release: https://pypi.python.org/pypi/NiBabel