PouchDB & other updates to README.md
Document in this branch that for CSharp-SQLite library may use DLL or rebuild from this source; document iOS is *always* using threadpool for background processing.
Merge branch 'common-src' into master-src: cs-sqlite-dll completely removed; updates to README.md
Merge branch 'master-src' into master-rc: cs-sqlite-dll completely removed; updates to README.md