Update README to use optional dependency over requirements.txt

docs/README.rst

@@ -26,7 +26,7 @@ the following tools are needed to build the documentation:
 In a conda environment, or a Python 3 ``venv``, you should be able to run::
 
   cd ipython
-  pip install -U -r docs/requirements.txt
+  pip install .[doc] -U
 
 
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