docs: add quick contributing instructions

CONTRIBUTING.md

@@ -0,0 +1,49 @@
+# Contributing to Postal
+
+This doc explains how to go about running Postal in development to allow you to make contributions to the project.
+
+## Dependencies
+
+You will need a MySQL database server to get started. Postal needs to be able to make databases within that server whenever new mail servers are created so the permissions that you use should be suitable for that.
+
+You'll also need Ruby. Postal currently uses Ruby 3.2.1. Install that using whichever version manager takes your fancy - rbenv, asdf, rvm etc.
+
+## Clone
+
+You'll need to clone the repository
+
+```
+git clone git@github.com:postalserver/postal
+```
+
+Once cloned, you can install the Ruby dependencies using bundler.
+
+```
+bundle install
+```
+
+## Configuration
+
+At present, configuration is handled using a config file. This lives in `config/postal/postal.yml`. An example configuration file is provided in `config/postal.example.yml`. This example is for development use only and not an example for production use.
+
+You'll also need a key for signing. You can generate one of these like this:
+
+```
+openssl genrsa -out config/postal/signing.key 2048
+```
+
+## Running
+
+The neatest way to run postal is to ensure that `./bin` is your `$PATH` and then use one of the following commands.
+
+* `bin/dev` - will run all components of the application using Foreman
+* `bin/postal` - will run the Postal binary providing access to running individual components or other tools.
+
+## Database initialization
+
+Use the commands below to initialize your database and make your first user.
+
+```
+postal initialize
+postal make-user
+```

config/postal.example.yml

@@ -0,0 +1,38 @@
+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