add(book): Describes custom Testnets in the Zebra book #78

Job	Run time
Build CI Docker / Build images	1s
Generate checkpoints mainnet / Run checkpoints-mainnet test	1s
Check if cached state disks exist for Mainnet / Check if cached state disks exist	1s
get block template / Run get-block-template test	1s
lightwalletd tip send / Run lwd-send-transactions test	1s
Zebra tip JSON-RPC / Run fully-synced-rpc test	1s
Test all	1s
Test checkpoint sync from empty state	1s
Test with fake activation heights	1s
Test CI default Docker config file / Test default-conf in Docker	1s
Test integration with lightwalletd	1s
Zebra checkpoint update / Run sync-past-checkpoint test	1s
Zebra tip update / Run update-to-tip test	1s
Test CI custom Docker config file / Test custom-conf in Docker	1s
lightwalletd tip / Run lwd-full-sync test	1s
lightwalletd GRPC tests / Run lwd-grpc-wallet test	1s
lightwalletd tip update / Run lwd-update-sync test	1s
submit block / Run submit-block test	2s
Test CI testnet Docker config file / Test default-conf in Docker	1s
	20s

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