Document preference for pip install over setup.py.

CONTRIBUTING.rst

@@ -20,7 +20,10 @@ Making Your First Change
 ========================
 
 First, install xonsh from source and open a xonsh shell in your favorite
-terminal application. See installation instructions for details.
+terminal application. See installation instructions for details, but it
+is recommended to do an 'editable' install via `pip'
+
+        $ pip install -e .
         
 Next, make a trivial change (e.g. ``print("hello!")`` in ``main.py``).
 
@@ -108,8 +111,7 @@ is open to interpretation.
 * Simple functions should have simple docstrings.
 * Lines should be at most 80 characters long. The 72 and 79 character
   recommendations from PEP8 are not required here.
-* All Python code should be compliant with Python 3.5+.  At some
-  unforeseen date in the future, Python 2.7 support *may* be supported.
+* All Python code should be compliant with Python 3.5+.  
 * Tests should be written with `pytest <https://docs.pytest.org/>`_ using a procedural style. Do not use
   unittest directly or write tests in an object-oriented style.
 * Test generators make more dots and the dots must flow!

docs/linux.rst

@@ -28,7 +28,7 @@ the following from the source directory,
 
 .. code-block:: console
 
-    $ python setup.py install
+    $ pip install .
 
 
 Debian/Ubuntu users can install xonsh from the repository with:

docs/osx.rst

@@ -41,7 +41,7 @@ the following from the source directory,
 
 .. code-block:: console
 
-    $ python3 setup.py install
+    $ pip3 install xonsh
 
 Extras for macOS
 ==================

docs/windows.rst

@@ -50,7 +50,7 @@ Now install xonsh:
 .. code-block:: bat
 
    > cd xonsh-master
-   > python setup.py install
+   > pip install .
 
 Next, run xonsh:
 

setup.py

@@ -196,7 +196,10 @@ def restore_version():
 
 
 class xinstall(install):
-    """Xonsh specialization of setuptools install class."""
+    """Xonsh specialization of setuptools install class.
+    For production, let setuptools generate the 
+    startup script, e.g: `pip installl .' rather than
+    relying on 'python setup.py install'."""
 
     def run(self):
         clean_tables()