Slight clarifications to README

Anything in `<>` is something you need to fill in.

As far as the first concern goes, I tried to break this in several ways, all of which failed:

```
function make_f()
    eval(:(f(x) = x+1))
end

function g(x)
    f(2*x)
end
```
works, as does
```
@eval begin
    $funcname(x) = x+1
end
```
where `funcname = :testme` is defined in the main `X` module. Also, functions with type-inference problems seem to work just fine:
```
function bad(sz::Vector{Int})
    A = Array(Float64, sz...)
    @show typeof(A)
    A
end

function good{N}(sz::NTuple{N,Int})
    A = Array(Float64, sz)
    @show typeof(A)
    A
end
```
I suspect I wouldn't be alone in not having a clear idea of where the problems are likely to crop up.

I'll revise this, just in the middle of some other stuff ATM.