Update README.md to reflect usage of GitLab

Also mention GitLab instead of GitHub when talking about contributing
to point potential contributors to our self-hosted GitLab instance
instead of GitHub
7 jobs for !1119 with fix-README in 1 minute and 12 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Lint
passed #14699
x86_64 docker-alpine
lint

00:00:27

 
  Build
passed #14704
aarch64 ci-build docker-alpine
build-aarch64

00:00:42

passed #14705
armv7 ci-build docker-alpine
build-armv7

00:00:43

passed #14703
ppc64le ci-build docker-alpine
build-ppc64le

00:00:33

passed #14702
s390x ci-build docker-alpine
build-s390x

00:00:35

passed #14701
x86 ci-build docker-alpine
build-x86

00:00:28

passed #14700
x86_64 ci-build docker-alpine
build-x86_64

00:00:34