Formulate more clearly in the readme the system-calls issue

[gnzlbg]

The readme currently states as Drawback:

• global allocator bypass: SliceDeque bypasses Rust's global allocator / it is its own memory allocator, talking directly to the OS. That is, allocating and growing SliceDeques always involve system calls, while a VecDeque backed-up by a global allocator might receive memory owned by the allocator without any system calls at all.

Until, as @bill-myers mentioned in #19 , the library starts caching previously-freed allocations for future reuse, this is going to remain a major drawback of the library and users should be aware of it.

This drawback should be explaining more clearly to avoid any misunderstandings.

[gnzlbg]

The reddit post explains the drawbacks a bit differently, maybe the explanations there are more clear?

https://www.reddit.com/r/rust/comments/7u45jv/slicedeque_a_doubleended_queue_that_derefs_into_a/