Add a page to the homepage for Hack Brexit Open Source Accelerator

[charlottebrf]

1. Create Open Source Accelerator page
2. Link page to the homepage nav bar
3. Add explanation of the Hack Brexit Open Source Accelerator, including links to T&Cs & Wikis
4. Direct visitors to Hack Brexit meet up group: https://www.meetup.com/Hack-Brexit/
5. Include slack sign up link

@eddiejaoude - sounds good to you?