do not forget to include the new opf as well

do not forget to include the new opf as well

Merge pull request Sigil-Ebook#12 from mizuOda/master

add CSSValidator plugin to Manage Plugins chapter

update User Guide for Sigil-1.6.0+

make guide consistent for 1.5.1 changes

update to epub3 with backwards compat ncx and guide

update user guide to final Sigil 1.5.0 version

update user guide to final Sigil 1.5.0 version

Add rudimentary documentation for new external xhtml editor button

Remove manifested images that were no longer being used