Add some development instructions to CONTRIBUTING.md (#1348)

CONTRIBUTING.md

+# Contributing
+
+Mail-in-a-Box is an open source project. Your contributions and pull requests are welcome.
+
+## Development
+
+To start developing Mail-in-a-Box, [clone the repository](https://github.com/mail-in-a-box/mailinabox) and familiarize yourself with the code.
+
+    $ git clone https://github.com/mail-in-a-box/mailinabox
+
+### Vagrant and VirtualBox
+
+We recommend you use [Vagrant](https://www.vagrantup.com/intro/getting-started/install.html) and [VirtualBox](https://www.virtualbox.org/wiki/Downloads) for development. Please install them first.
+
+With Vagrant set up, the following should boot up Mail-in-a-Box inside a virtual machine:
+
+    $ vagrant up --provision
+
+_If you're seeing an error message about your *IP address being listed in the Spamhaus Block List*, simply uncomment the `export SKIP_NETWORK_CHECKS=1` line in `Vagrantfile`. It's normal, you're probably using a dynamic IP address assigned by your Internet provider–they're almost all listed._
+
+### Modifying your `hosts` file
+
+After a while, Mail-in-a-Box will be available at `192.168.50.4` (unless you changed that in your `Vagrantfile`). To be able to use the web-based bits, we recommend to add a hostname to your `hosts` file:
+
+    $ echo "192.168.50.4 mailinabox.lan" | sudo tee -a /etc/hosts
+
+You should now be able to navigate to https://mailinabox.lan/admin using your browser. There should be an initial admin user with the name `me@mailinabox.lan` and the password `12345678`.
+
+### Making changes
+
+Your working copy of Mail-in-a-Box will be mounted inside your VM at `/vagrant`. Any change you make locally will appear inside your VM automatically.
+
+Running `vagrant up --provision` again will repeat the installation with your modifications.
+
+Alternatively, you can also ssh into the VM using:
+
+    $ vagrant ssh
+
+Once inside the VM, you can re-run individual parts of the setup like in this example:
+
+    vm$ cd /vagrant
+    vm$ sudo setup/owncloud.sh # replace with script you'd like to re-run
+
+### Tests
+
+Mail-in-a-Box needs more tests. If you're still looking for a way to help out, writing and contributing tests would be a great start!
+
 ## Public domain
 
 This project is in the public domain. Copyright and related rights in the work worldwide are waived through the [CC0 1.0 Universal public domain dedication][CC0]. See the LICENSE file in this directory.

README.md

@@ -82,6 +82,12 @@ For help, DO NOT contact me directly --- I don't do tech support by email or twe
 
 Post your question on the [discussion forum](https://discourse.mailinabox.email/) instead, where me and other Mail-in-a-Box users may be able to help you.
 
+Contributing and Development
+----------------------------
+
+Mail-in-a-Box is an open source project. Your contributions and pull requests are welcome. See [CONTRIBUTING](CONTRIBUTING.md) to get started. 
+
+
 The Acknowledgements
 --------------------