Add installing with pip from git (#15366)

It is also possible to install with pip from the latest development branch in git. Not sure about the wording tho.

docsite/rst/intro_installation.rst

@@ -323,6 +323,10 @@ Then install Ansible with [1]_::
 
    $ sudo pip install ansible
    
+Or if you are looking for the latest development version::
+    
+    pip install git+git://github.com/ansible/ansible.git@devel
+
 If you are installing on OS X Mavericks, you may encounter some noise from your compiler.  A workaround is to do the following::
 
    $ sudo CFLAGS=-Qunused-arguments CPPFLAGS=-Qunused-arguments pip install ansible