Add README.md to line-index crate

[homersimpsons]

I recently came across https://crates.io/crates/line-index, but the page looks rather empty.

The short-description explains a bit what is the purpose of this crate:

Maps flat TextSize offsets to/from (line, column) representation

But I think a README.md file explaining what is the purpose of this crate and how to use it may help contributors and other people using it.

Crate source: https://github.com/rust-lang/rust-analyzer/tree/master/lib/line-index

[lnicola]

Good idea, do you want to file a PR?

[homersimpsons]

do you want to file a PR?

I would like to fill a PR, but does not really know anything about this crate.

If you don't have time to create it, I will dig a bit in the code to understand and document it.