Meetup 21

Prepare the announcement of Thessaloniki Ruby Meetup 21:

* [x] Find the date and check with George for meeting room availability
* [x] Create file named YYYYMMDD-meetup-NN in the meetups directory
* [x] Fill in a nice title
* [x] Fill in a summary of the presentation
* [x] Fill in the name of the speaker as a link to their GitHub account
* [x] Fill in a short biography of the speaker
* [x] 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 thessrbNN to the permanent link of this file
* [ ] Go crazy announcing the meetup on Twitter, Facebook, etc.