Fix README flaws

docs/index.rst

@@ -4,7 +4,7 @@
    contain the root `toctree` directive.
 
 Welcome to parsedmarc's documentation!
-=======================================
+======================================
 
 |Build Status|
 
@@ -151,6 +151,37 @@ Please report bugs on the GitHub issue tracker
 
 https://github.com/domainaware/parsedmarc/issues
 
+Installation
+============
+
+``parsedmarc`` works with Python 2 or 3, but Python 3 is preferred.
+
+On Debian or Ubuntu systems, run:
+
+.. code-block:: bash
+
+    $ sudo apt-get install python3-pip
+
+
+Python 3 installers for Windows and macOS can be found at https://www.python.org/downloads/
+
+To install or upgrade to the latest stable release of ``parsedmarc`` on macOS or Linux, run
+
+.. code-block:: bash
+
+    $ sudo -H pip3 install -U checkdmarc
+
+Or, install the latest development release directly from GitHub:
+
+.. code-block:: bash
+
+    $ sudo -H pip3 install -U git+https://github.com/domainaware/parsedmarc.git
+
+.. note::
+
+    On Windows, ``pip3`` is ``pip``, even with Python 3. So on Windows, simply
+    substitute ``pip`` as an administrator in place of ``sudo pip3``, in the above commands.
+
 API
 ===