README.md: Shift Index discussion to the external-backend section

We use Index for third-party search engines.  For the 'sqlalchemy'
special case, we use SQLAlchemyIndex.
README.md: Document --preload for the SQLAlchemy search index
avoid using same variable twice in the same function
Merge pull request #659 from misho-kr/master

avoid using a variable twice in the same function
Merge pull request #655 from wking/document-preload

README.md: Document --preload for the SQLAlchemy search index