Update README with notice re: MySQL, SQLite vs. LatestEntriesPlugin

Also, add more complete info re: available plugins and minor spelling, formatting…
Add some narrative about lead-in vs. body and how to add the primary content to the blog post.
Fix some typos in the previous commit
Merge branch 'update_readme' of github.com:mkoistinen/aldryn-blog into mkoistinen-update_readme
simplifies db support message