docs: add quick contributing instructions

config/postal.example.yml

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+web:
+  host: postal.example.dev
+  protocol: https
+
+web_server:
+  bind_address: 0.0.0.0
+  port: 4010
+
+smtp_server:
+  port: 2525
+
+logging:
+  rails_log: true
+  stdout: true
+
+main_db:
+  host: 127.0.0.1
+  username: root
+  password: 
+  database: postal
+
+message_db:
+  host: 127.0.0.1
+  username: root
+  password: 
+  prefix: postal
+
+smtp:
+  host: 127.0.0.1
+  port: 2525
+  username:
+  password:
+  from_name: Postal
+  from_address: postal@yourdomain.com
+
+rails:
+  environment: development
+  secret_key: 7f27856d26e864bafd49d0df37ad3d1339086e86ef0447e0f1814dde5277452fea97dab9e3aad6dfa11bfe359c82ce302d97bf1e58f6103c4408e4fbad4eeccf