Update docs regarding building dulwich without c bindings

See: #1093.

Currently, both --global-option=--pure and --build-option=--pure
work (when set in --config-settings).  However, using --global-option
in this way is considered deprecated:

https://setuptools.pypa.io/en/latest/history.html#id139

https://github.com/pypa/setuptools/pull/3380

--no-binary is needed to prevent pip installing the pre-built wheels
from pypi.org.

README.rst

@@ -26,13 +26,13 @@ If you don't want to install the C bindings, specify the --pure argument to setu
 
 or if you are installing from pip::
 
-    $ pip install dulwich --global-option="--pure"
+    $ pip install --no-binary dulwich dulwich --config-settings "--build-option=--pure"
 
-Note that you can also specify --global-option in a
-`requirements.txt <https://pip.pypa.io/en/stable/reference/pip_install/#requirement-specifiers>`_
+Note that you can also specify --build-option in a
+`requirements.txt <https://pip.pypa.io/en/stable/reference/requirement-specifiers/>`_
 file, e.g. like this::
 
-    dulwich --global-option=--pure
+    dulwich --config-settings "--build-option=--pure"
 
 Getting started
 ---------------