add a meaningful readme file

10 jobs for feature/readme in 9 minutes and 3 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Test
passed debian:10 clang-7-libcpp-17 #20756

00:03:54

passed debian:10 gcc-7-17 #20757

00:04:43

passed debian:10 gcc-8-noassert-17 #20755

00:05:04

passed debian:11 gcc-9-20 #20754

00:05:09

passed ubuntu:18.04 clang-6-17 #20758

00:03:51

failed debian:10 clang-7-libcpp-17 #20741

00:03:11

failed debian:10 gcc-7-17 #20742

00:03:31

failed debian:10 gcc-8-noassert-17 #20740

00:03:34

failed debian:11 gcc-9-20 #20739

00:03:29

failed ubuntu:18.04 clang-6-17 #20743

00:03:22