chore(README.md): add pip in doc (close #222)

5 jobs for <span class="ref-name">feature/update-readme</span> in 45 minutes and 42 seconds (queued for 55 seconds)
Status Job ID Name Coverage
  Test
passed #4410
docker
job-debian9-ninja-clang

00:04:59

passed #4409
docker
job-debian9-ninja-gcc

00:02:57

passed #4411
macosx
job-macosx-ninja

00:09:16

passed #4412
macosx
job-macosx-xcode

00:19:10

passed #4413
windows
job-windows-vs2015

00:12:16