readme: Clarify build instructions

3 jobs for wip/mschraal/update-readme in 7 minutes and 40 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Check
passed #10167
pep8

00:02:55

passed #10166
pyflakes

00:02:19

 
  Build
passed #10168
build

00:04:45