Document changes to rewrite middleware in wai-extra
Document changes to rewrite middleware in wai-extra

* Also expand documentation with detailed instructions for using the module

Then,

* Fix documentation based on feedback from @MaxGabriel

sordina/wai,kazu-yamamoto/wai,sordina/wai,yesodweb/wai,tolysz/wai,tolysz/wai,kazu-yamamoto/wai,creichert/wai,creichert/wai,tolysz/wai,sordina/wai,tolysz/wai,sordina/wai,yesodweb/wai