manual build corrections in readme.md

[novski]

I figured out some issues in manual build, hope this helps others to build manualy faster...
Session variable is needed on raspberrypi 3+. Allthroug im not shure if it's the best way to go...

[coveralls]

Coverage decreased (-0.09%) to 99.569% when pulling d1c4ed3 on novski:master into c42392d on cdubz:master.

[cdubz]

Thanks for the PR.

Interesting note - the sudo -H pip install pipenv command does not work when I tested walking through this on a Pi with a fresh install of raspbian strecth. I wonder if the alias does not work if python2 pip isn't installed at all (it was not in my test environment)?

Anyway, could you push another commit removing the alias pip=pip3 line from step 2 and changing step 3 to sudo -H pip3 install pipenv? I think that is the only place that pip3 is needed. I'll accept this PR with those changes.