Add contribution guide

2 jobs for master in 10 minutes and 23 seconds
Name Stage Failure
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build Build There is an unknown failure, please try again
Err http://security.debian.org jessie/updates/main amd64 Packages

Err http://security.debian.org jessie/updates/main amd64 Packages
Unable to connect to security.debian.org:http: [IP: 2607:ea00:101:3c0b::1deb:215 80]
Fetched 10.6 MB in 10min 18s (17.2 kB/s)
W: Failed to fetch http://security.debian.org/dists/jessie/updates/main/binary-amd64/Packages Unable to connect to security.debian.org:http: [IP: 2607:ea00:101:3c0b::1deb:215 80]

E: Some index files failed to download. They have been ignored, or old ones used instead.
ERROR: Build failed: exit code 1