Add project Makefile

[jesusvazquez]

By running make all you'll get a binary file couchdb-admin that will run under linux and amd64 arch.

In case you want to quickly test the binary on macOS run make darwin which will create the file couchdb-admin-darwin-amd64.

./couchdb-admin-darwin-amd64
NAME:
   CouchDB 2 Admin tool - Easily operate a CouchDB 2 cluster

USAGE:
   $ couchdb-admin [COMMAND] [OPTIONS]

VERSION:
   0.1.0

AUTHOR:
   Carlos Alonso <[email protected]>

COMMANDS:
     describe_db               Get a db's shards placement info
     replicate                 Replicate a database's shard into a node not containing it already
     add_node                  Join a node into the cluster
     describe_cluster          Get information of the cluster's nodes
     create_db                 Create a new database
     remove_replica            Remove a shard from a particular node
     disable_maintenance_mode  Disable a node's maintenance mode
     set_config                Set a config value of a particular node
     remove_node               Remove a node from the cluster
     help, h                   Shows a list of commands or help for one command

GLOBAL OPTIONS:
   --server value    The server to connect to (default: "127.0.0.1")
   --admin value     Admin of the DB (default: "admin")
   --password value  Password for the db's admin (default: "password")
   --help, -h        show help
   --version, -v     print the version

[calonso]

LGTM. Add a small building section on the README.md before merging, please.