Write contributor guidelines

[msmichellegar]

We need some contributor guidelines, explaining how to contribute to the Node Girls website. Given we are now expanding into a number of international chapters, I believe the guidelines need to cover the following tasks:

• How to add a new chapter
• How to add a new mentor or organiser
• How to add a new event

Contributor guidelines should also cover the following:

• Making pull requests
• Raising issues
• Commit messages
• Who to ask for help
• Code of conduct