Update docs README with build instructions using tox
Update docs README with build instructions using tox

None of the existing commands built the docs properly for me (I had to
revert the changes in 901892d to docs/conf.py to get them to build
properly, and even then no tooltips displayed).

Building them with tox worked for me, but other developers say they
can still use the original method, so the docs now contain both.

pablohoffman/scrapy,starrify/scrapy,elacuesta/scrapy,pablohoffman/scrapy,dangra/scrapy,pablohoffman/scrapy,pawelmhm/scrapy,scrapy/scrapy,scrapy/scrapy,elacuesta/scrapy,scrapy/scrapy,starrify/scrapy,pawelmhm/scrapy,pawelmhm/scrapy,dangra/scrapy,dangra/scrapy,starrify/scrapy,elacuesta/scrapy