Update development version installation instructions

The current instructions have been invalid since
b32ec53 (move configuration into
`pyproject.toml`).

README.rst

@@ -43,7 +43,7 @@ To clone from github and install the master branch::
 
     git clone https://github.com/spacetelescope/gwcs.git
     cd gwcs
-    python setup.py install
+    pip install --editable .
 
     
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