Update README.md

[cjprybol]

https://github.com/JuliaWeb/Requests.jl/issues/166

julia> using Requests

julia> html = "https://github.com/johnmyleswhite/RDatasets.jl/raw/master/data/MASS/SP500.csv.gz"
"https://github.com/johnmyleswhite/RDatasets.jl/raw/master/data/MASS/SP500.csv.gz"

julia> Requests.read(get(html))
24348-element Array{UInt8,1}:
 0x1f
 0x8b
 0x08
 0x08
 0xc7
 0x80
 0xcb
 0x52
 0x02
 0x03
 0x53
 0x50
 0x35
 0x30
 0x30
 0x2e
 0x63
 0x73
 0x76
 0x00
 0x6d
 0x9d
 0x59
 0x96
 0x2c
 0x49
 0x92
    ⋮
 0x81
 0xc4
 0x09
 0x7f
 0x9d
 0x04
 0x26
 0x08
 0x2d
 0x8a
 0x1c
 0x10
 0xbe
 0xc8
 0x6f
 0xff
 0xfc
 0x1f
 0x8c
 0x16
 0x7c
 0x91
 0x5c
 0xc6
 0x00
 0x00

julia> Requests.readbytes(get(html))
ERROR: UndefVarError: readbytes not defined