Amended install instructions in README.rst
Amended install instructions in README.rst

Purpose: helps users avoid several installation errors
Changes:
- added instruction to install requirements using pip
- added instruction to install required packages build-essential,
libssl-dev, libffi-dev

DocImpact
Closes-Bug: #1503094
Closes-Bug: #1503095

Change-Id: Iebdb25f259095be6e7e237c8a683f657ffeab92a

ramineni/my_congress,ramineni/my_congress,openstack/congress,openstack/congress,ramineni/my_congress,ramineni/my_congress