Add step 0 to our Curriculum

[kfklein15]

Background We remembered about step 0, which is to invite them to ITD Slack, late in the game, so this step isn't included on our Curriculum, causing confusion.

Acceptance criteria

• Add the step to our page: https://cyf-itd.netlify.app/steps/ as step zero
• Do NOT add the link to actually accept invite to Slack, otherwise people that have not singed-up can join Slack and that would generate confusion.