DOC update changelog
DOC update changelog

* changes from recently merged pull requests
* more highlights
* re-organized headers
* Selector API changes

pablohoffman/scrapy,scrapy/scrapy,elacuesta/scrapy,dangra/scrapy,starrify/scrapy,scrapy/scrapy,finfish/scrapy,eLRuLL/scrapy,pawelmhm/scrapy,dangra/scrapy,pablohoffman/scrapy,finfish/scrapy,scrapy/scrapy,starrify/scrapy,elacuesta/scrapy,pawelmhm/scrapy,wujuguang/scrapy,finfish/scrapy,pawelmhm/scrapy,pablohoffman/scrapy,elacuesta/scrapy,wujuguang/scrapy,wujuguang/scrapy,dangra/scrapy,eLRuLL/scrapy,eLRuLL/scrapy,starrify/scrapy