README: update, add sections "build configuration" and "contributing"

3 jobs for master in 21 seconds
Status Name Job ID Coverage
  Test
passed mirror #4151

00:00:21

failed python2 #4149

00:00:09

passed python3 #4150

00:00:18

 
Name Stage Failure
failed
python2 Test There is an unknown failure, please try again
Running on runner-b582349c-project-2-concurrent-0 via qclab-hpc1...
Fetching changes...
HEAD is now at 64aaf0b tests: remove forgotten print()
From https://gitlab.kwant-project.org/kwant/tinyarray
64aaf0b..8e81957 master -> origin/master
Checking out 8e819577 as master...
$ python2 setup.py build_ext -i
/bin/bash: line 44: python2: command not found
ERROR: Build failed: exit code 1