Readme Driven Development

Write your Readme first. 

First. As in, before you write any code or tests or behaviors or stories or ANYTHING. 

By writing your Readme first you give yourself some pretty significant advantages:

• Most importantly, you're giving yourself a chance to think through the project without the overhead of having to change code every time you change your mind about how something should be organized or what should be included in the Public API. Remember that feeling when you first started writing automated code tests and realized that you caught all kinds of errors that would have otherwise snuck into your codebase? That's the exact same feeling you'll have if you write the Readme for your project before you write the actual code.
• As a byproduct of writing a Readme in order to know what you need to implement, you'll have a very nice piece of documentation sitting in front of you. You'll also find that it's much easier to write this document at the beginning of the project when your excitement and motivation are at their highest. Retroactively writing a Readme is an absolute drag, and you're sure to miss all kinds of important details when you do so.
• If you're working with a team of developers you get even more mileage out of your Readme. If everyone else on the team has access to this information before you've completed the project, then they can confidently start work on other projects that will interface with your code. Without any sort of defined interface, you have to code in serial or face reimplementing large portions of code.
• It's a lot simpler to have a discussion based on something written down. It's easy to talk endlessly and in circles about a problem if nothing is ever put to text. The simple act of writing down a proposed solution means everyone has a concrete idea that can be argued about and iterated upon.

- More here by Tom Preston-Werner