.github: add contributing guidelines and security policy #73

Job	Run time
container (httpd) / build (arm64)	3m 48s
container (nftables) / build (arm64)	4m 14s
container (ntpd) / build (arm64)	5m 20s
container (neofetch) / build (arm64)	6m 56s
container (ntpd) / build (amd64)	5m 4s
container (system) / build (amd64)	13m 4s
container (nftables) / build (amd64)	4m 15s
container (httpd) / build (amd64)	4m 13s
container (neofetch) / build (amd64)	7m 18s
container (system) / build (arm64)	13m 37s
container (httpd) / push	37s
container (nftables) / push	22s
container (ntpd) / push	24s
container (neofetch) / push	37s
container (system) / push	46s
	1h 10m 35s

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