update README.rst to include install instructions for RHEL

To install this package:

For Debian/Ubuntu,

.. code-block:: bash

   $ sudo apt install -y redis-server pipx   # redis is used to cache block sha1sum

   $ pipx install rd-api

For RHEL,

.. code-block:: bash

   $ sudo dnf install redis python3-pip

   $ sudo systemctl start redis

   $ pip install --user rd-api

To run rd-api

.. code-block:: bash

   $ rd-api --help

   $ ~/.local/bin/rd-api --help     # if ~/.local/bin/ is not in PATH

To install this package:

For Debian/Ubuntu,

.. code-block:: bash

   $ sudo apt install -y pipx

   $ pipx install rd

For RHEL,

.. code-block:: bash

   $ sudo dnf install python3-pip

   $ pip install --user rd

To run rd

.. code-block:: bash

   $ rd --help

   $ ~/.local/bin/rd --help     # if ~/.local/bin/ is not in PATH