readme: now working on edX, maintenance mode (#2505)

Also corrected sandbox link.  DB Class in past tense.
Merge branch 'master' of github.com:Stanford-Online/class2go into sef-2505-edx-readme-homepage
added edX announcement to landing.html
readme: tighten up the language a wee bit
Merge branch 'sef-2505-edx-readme-homepage' of github.com:Stanford-Online/class2go into sef-2505-edx-readme-homepage
landing: link to both announce and forum post (#2505)
Merge pull request #2513 from Stanford-Online/sef-2505-edx-readme-homepage

edX on landing page and readme -- MASTER