Added more complete instructions for setting up a development environment for Home Assistant. Including details of dependencies, setting up venv. This is based on notes captured from setting up a dedicated hass dev environment on Debian 8.6.0

Updating development environment setup guide

Updated as a result of feedback on pull request: #1804
Adding reminder to activate virtual environment before running setup along with adding reference to indicate apt-get may not be applicable to distro in use.

Remove statement about running hass with --open-ui and what it does (wrongly stated). Based on feedback from @balloob.

As of Python 3.6 pyvenv has been deprecated in favour of python3 -m venv.

Further changes may be required following resolution of discussion around setting up venv in a sub directory vs at the top level of the project.

Given change [#5507](home-assistant/core#5507) only attempts to setup the frontend if nvm is present. It makes sense to remove the details from here, since that was mainly around avoiding errors in the first time setup scenario. Instead I've adjusted the text to point to the FrontEnd Dev section.

yum install commands needs to be split so that EPEL repo is added first ahead of attempting to install other dependencies.

…nt.github.io into next

# Conflicts:
#	source/developers/development_environment.markdown