Expand crate-level documentation

[sfackler]

The root of a crate's documentation is the first thing a prospective user sees, and should provide an introduction to both the concepts the library deals with as well as quick examples of common usage patterns. See https://github.com/jonathandturner/rfcs/blob/b4ff67a2406651372c72fe7af856c3af62846e67/text/0000-api-doc-frontpage-styleguide.md for the kinds of things the crate root docs should discuss.

[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.

[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.