Update README for use python3

And small change for `long_description_content_type` missing.

Change-Id: I042a319bfc5009ce625565effa7ccf634222be28

README.rst

@@ -48,12 +48,12 @@ Getting Started
 
 OpenStack Client can be installed from PyPI using pip::
 
-    pip install python-openstackclient
+    python3 -m pip install python-openstackclient
 
 There are a few variants on getting help.  A list of global options and supported
 commands is shown with ``--help``::
 
-   openstack --help
+    openstack --help
 
 There is also a ``help`` command that can be used to get help text for a specific
 command::
@@ -64,11 +64,9 @@ command::
 If you want to make changes to the OpenStackClient for testing and contribution,
 make any changes and then run::
 
-    python setup.py develop
+    git clone https://opendev.org/openstack/python-openstackclient
-
+    cd python-openstackclient
-or::
+    python3 -m pip install -e .
-
-    pip install -e .
 
 Configuration
 =============

setup.cfg

@@ -3,6 +3,7 @@ name = python-openstackclient
 summary = OpenStack Command-line Client
 description_file =
     README.rst
+description-content-type = text/x-rst
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/python-openstackclient/latest/