Updated installation instructions to use pip install

Author: kaklise

documentation/installation.rst

@@ -18,9 +18,9 @@ To install the development branch of Chama from source using git::
 
 	git clone https://github.com/sandialabs/chama
 	cd chama
-	python setup.py install
+	python -m pip install .
 
-Developers should build Chama using the setup.py 'develop' option.
+Developers should build Chama using ``python -m pip install -e .``
 
 Dependencies
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