Update README.rst, add requirement: attrs (#2855)
Update README.rst, add requirement: attrs (#2855)

README file is difference with docs, with requirements section. 

README file missing `attrs` package, but it is required
arthurdarcet/aiohttp,KeepSafe/aiohttp,rutsky/aiohttp,KeepSafe/aiohttp,rutsky/aiohttp,rutsky/aiohttp,arthurdarcet/aiohttp,KeepSafe/aiohttp,arthurdarcet/aiohttp