Expand crate-level documentation

[danburkert]

Expand crate-level documentation

[danburkert]

@burdges what do you mean by a table? To get a good sense of what people are using the crate for I'd recommend looking at https://crates.io/crates/memmap/reverse_dependencies. fst and sanakirja might be particularly interesting if you are looking to use mmap for file reading/writing.

[sfackler]

Note that this should be sequenced after #33.

Particular care should be taken to discuss what exactly the safety considerations are around the unsafe constructors.

[burdges]

Are there any nice to read examples or discussions of people using memmap to back large tables? Ideally in Rust using this crate, but even in C if needs be. I want to get an idea for what I don't know about using memmap before using it.