add pip installation instructions to README.md

16 jobs for master in 5 minutes and 19 seconds
Status Job ID Name Coverage
  Build
passed #44793
Linux
build-2.7-linux

00:01:28

passed #44794
Apple
build-2.7-macos

00:02:13

passed #44791
Linux
build-3.5-linux

00:01:45

passed #44792
Apple
build-3.5-macos

00:02:14

passed #44790
VS2017
build-3.6-VS2017

00:01:51

 
  Test
passed #44798
Linux
test-2.7-linux

00:00:08

passed #44799
Apple
test-2.7-macos

00:00:11

passed #44796
Linux
test-3.5-linux

00:00:09

passed #44797
Apple
test-3.5-macos

00:00:13

passed #44795
VS2017
test-3.6-VS2017

00:00:29

 
  Deploy
passed #44803
Linux
deploy-2.7-linux

00:00:01

passed #44804
Apple
deploy-2.7-macos

00:00:01

passed #44801
Linux
deploy-3.5-linux

00:00:01

passed #44802
Apple
deploy-3.5-macos

00:00:01

passed #44800
VS2017
deploy-3.6-VS2017

00:00:04

passed #44805
Linux
deploy-documentation

00:00:19