Valid categories in toml descriptions are out of date

[carols10cents]

ThreadSafety/thread-safety is a valid category https://docs.rs/rustsec/latest/rustsec/advisory/enum.Category.html

But thread-safety doesn't appear in https://github.com/rustsec/advisory-db/blob/f414b4d1ff5df405ea74240bc8fc2e4ce5f0d6c3/README.md#L78-L80 or https://github.com/rustsec/advisory-db/blob/f414b4d1ff5df405ea74240bc8fc2e4ce5f0d6c3/EXAMPLE_ADVISORY.md#L7-L8

[tarcieri]

We should probably just give a couple few example categories, but then link to the rustdoc rather than trying to explicitly maintain a list of categories here that could go out of date in the future if more categories are added.

That would also help inform people of the category descriptions.