DOC: update "installation" section (#36)

* DOC: update "installation" section

* DOC: Remove the --user flag from the instructions

Author: mgeier

CONTRIBUTING.rst

@@ -15,7 +15,7 @@ newest development version (a.k.a. "master") with Git::
 
    git clone https://github.com/spatialaudio/python-rtmixer.git --recursive
    cd python-rtmixer
-   python3 -m pip install -e . --user
+   python3 -m pip install -e .
 
 ... where ``-e`` stands for ``--editable``.
 
@@ -51,7 +51,7 @@ pages locally using Sphinx and check if they look OK.
 Initially, you might need to install a few packages that are needed to build the
 documentation::
 
-   python3 -m pip install -r doc/requirements.txt --user
+   python3 -m pip install -r doc/requirements.txt
 
 To (re-)build the HTML files, use::
 

doc/installation.rst

@@ -1,19 +1,17 @@
 Installation
 ============
 
-On Windows, macOS, and Linux you can install a precompiled wheel with::
+On Windows, macOS and Linux you can install a precompiled "wheel" package with::
 
     python3 -m pip install rtmixer
 
 This will install ``rtmixer`` and its dependencies, including ``sounddevice``.
 
+Depending on your Python installation,
+you may have to use ``python`` instead of ``python3``.
+If you have installed the module already,
+you can use the ``--upgrade`` flag to get the newest release.
+    
 .. note:: On Linux, to use ``sounddevice`` and ``rtmixer`` you will need to
           have PortAudio installed, e.g. via ``sudo apt install libportaudio2``.
           On other platforms, PortAudio comes bundled with ``sounddevice``.
-
-Developers can install in editable mode with some variant of::
-
-    git clone https://github.com/spatialaudio/python-rtmixer
-    cd python-rtmixer
-    git submodule update --init
-    python3 -m pip install -e .