Update edX submission instructions

[brianyu28]

When we debut the new check50 and integrate it with CS50.me, we'll need to...

For pset6, replace the submit50 slug cs50/2017/x/pset6 with:

• C-to-Python slugs:
  • cs50/2017/x/python/mario/less
  • cs50/2017/x/python/mario/more
  • cs50/2017/x/python/greedy
  • cs50/2017/x/python/credit
  • cs50/2017/x/python/caesar
  • cs50/2017/x/python/vigenere
  • cs50/2017/x/python/crack
• Sentiments slugs:
  • cs50/2017/x/smile
  • cs50/2017/x/tweets
  • cs50/2017/x/sentiments

For pset0, if we're going to have students submit via GitHub:

• instructions for creating a cs50/2017/x/scratch branch, and uploading their .sb file.

For all psets:

• mandate the cs50/ prefix if we're going to get rid of the assumption of cs50/ at the beginning

CC @dmalan @crossroads1112

[dmalan]

@brianyu28, closable now?

[brianyu28]

Yep!