Add and Update README's for the various Social Projects. [#2934841]

Social API projects have some serious documentation issues. Many of the newer projects do not have READMEs and documentation guides. The first step is going to be to create and update READMEs for nearly all of them. This issue will act as a ROADMAP for the 3 main issues have been created, and each of these main issues has its own collection of child issues.

- Social Auth main issue: https://www.drupal.org/node/2934724

-Social Post main issue: https://www.drupal.org/node/2934835/

-Social Widgets issue: https://www.drupal.org/project/2761405/issues/2934832