Meetup 21

Prepare the announcement of Thessaloniki Ruby Meetup 21:

* [ ] Find the date and check with George for meeting room availability
* [ ] Create file named YYYYMMDD-meetup-NN in the meetups directory
* [ ] Create a lanyrd event for attendance
* [ ] Fill in a nice title
* [ ] Fill in a summary of the presentation
* [ ] Fill in the name of the speaker as a link to their GitHub account
* [ ] Fill in a short biography of the speaker
* [ ] Add some references to resources about the main topic of the presentation under the "Notes" section as a bullet list
* [ ] When all above are checked off, merge this
* [ ] Create gitio URL with code thessrb21 to the permanent link of this file
* [ ] Go crazy announcing the meetup on Twitter, Facebook, etc.