Readme install instructions issue

[JasonIamAUnixAdmin]

Instructions currently are:

git clone https://github.com/errbotio/err-backend-slackv3
# to get a specific release use `--branch <release-tag>`, e.g. `--branch v0.1.0`
git clone --depth 1 https://github.com/errbotio/err-backend-slackv3
pip install .

Should be more like

cd /opt/errbot/backend
git clone https://github.com/errbotio/err-backend-slackv3
#or to get a specific release use `--branch <release-tag>`, e.g. `--branch v0.1.0`
#git clone --depth 1 https://github.com/errbotio/err-backend-slackv3
cd err-backend-slackv3 && pip install .

Also the step before lists:

BOT_EXTRA_BACKEND_DIR="/opt/errbot/backend"

with instructions as written that needs to be:

BOT_EXTRA_BACKEND_DIR="/opt/errbot/backend/err-backend-slackv3"