Update variable scope section in ReadMe instructing to use a let block

[Tortar]

Should superseed #61 and fix #66

both of this work with the new suggestion:

using Suppressor
g = let
        g = 1
        @suppress g += 1
        g
    end

module M
    using Suppressor
    g = let
            g = 1
            @suppress g += 1
            g
        end
end

since now

julia> g
2

julia> Main.M.g
2

[Tortar]

Actually another possibility is to say that @supress should be used only one expression at a time with rebinding e.g.

julia> using Suppressor

julia> f = @suppress function f end; f
f (generic function with 0 methods)

julia> f
f (generic function with 0 methods)