update readme to real async get

[acrogenesis]

Fix #8

[juliusbierk]

Probably even more correct to do

from requests_threads import AsyncSession

session = AsyncSession(n=100)

async def _main():
    rs = []
    for _ in range(100):
        rs.append(asyncio.ensure_future(session.get('http://httpbin.org/get')))
    for i in range(100):
        rs[i] = await rs[i]
    print(rs)

if __name__ == '__main__':
    session.run(_main)

See https://docs.python.org/3/library/asyncio-future.html#asyncio.ensure_future