Removed unnecessary step
Fixed bug where slot shows as available

if no phone number was supplied. This was caused because a slot is only
considered taken if three details (name, email and phone) are saved in
that slot. I now specifically write “Not provided” to the slot, if no
phone number is supplied.
On save, add email to exclusion emails

to prevent one individual signing up multiple times
Convert $exclusion_emails back to string on set

You will need to write out a string, so the array should be converted
to string on set
New method addExclusionEmails()

Adds a bunch of emails to the $exclusion_emails array and converts to
string for writing out.
Changed wording

to reflect the fact that those already signed up to *this* experiment
are also ineligible.
Added basic description to the readme