Add 2020 conferences

[ledell]

Update README for 2020:

• Remove the block of 2019 conferences from the main README (and if they are not present on https://github.com/WiMLDS/conferences/blob/master/2019_listings.md please add them, otherwise you can just delete).
• Move the 2020 block up to the top of the README.
• To fill in 2020, duplicate the list of 2019 conferences from the repo with new dates.
• Also can add some from here: https://tryolabs.com/blog/machine-learning-deep-learning-conferences/

We also have the link to these on the README. Maybe we can just cherry pick the most relevant/good ones for our list and then provide the links for reference.

• KDD List of International Conferences
• Women in Machine Learning Workshop
• PyData conferences