Add more documentation about crates.io's database needs
Add more documentation about crates.io's database needs

Fixes #541.

rust-lang/crates.io,steveklabnik/crates.io,steveklabnik/crates.io,rust-lang/crates.io,steveklabnik/crates.io,rust-lang/crates.io,steveklabnik/crates.io,rust-lang/crates.io