DOC update "Contributing" docs:
DOC update "Contributing" docs:

* suggest Stack Overflow for Scrapy usage questions;
* encourage users to submit test-only pull requests with reproducable examples;
* encourage users to pick up stalled pull requests;
* we don't use AUTHORS file as a main acknowledgement source;
* suggest using Sphinx autodocs extension

starrify/scrapy,wujuguang/scrapy,Ryezhang/scrapy,finfish/scrapy,umrashrf/scrapy,dangra/scrapy,finfish/scrapy,ssteo/scrapy,pablohoffman/scrapy,eLRuLL/scrapy,eLRuLL/scrapy,wujuguang/scrapy,kmike/scrapy,Ryezhang/scrapy,ArturGaspar/scrapy,kmike/scrapy,scrapy/scrapy,umrashrf/scrapy,starrify/scrapy,dangra/scrapy,ArturGaspar/scrapy,elacuesta/scrapy,Ryezhang/scrapy,ssteo/scrapy,pawelmhm/scrapy,finfish/scrapy,ssteo/scrapy,eLRuLL/scrapy,starrify/scrapy,pawelmhm/scrapy,dangra/scrapy,pablohoffman/scrapy,elacuesta/scrapy,scrapy/scrapy,pablohoffman/scrapy,kmike/scrapy,scrapy/scrapy,ArturGaspar/scrapy,elacuesta/scrapy,wujuguang/scrapy,umrashrf/scrapy,pawelmhm/scrapy