Auto merge of rust-lang#16302 - homersimpsons:patch-1, r=lnicola

line-index: Create README.md

Fixes rust-lang#16180

lib/line-index/README.md

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+# line-index
+
+This crate is developed as part of `rust-analyzer`.
+
+line-index is a library to convert between text offsets and corresponding line/column coordinates.
+
+## Installation
+
+To add this crate to a project simply run `cargo add line-index`.
+
+## Usage
+
+The main structure is `LineIndex`.
+
+It is constructed with an UTF-8 string, but also supports UTF-16 and UTF-32 offsets.
+
+### Example
+
+```rust
+use line_index::LineIndex;
+
+let line_index = LineIndex::new("This is a\nmulti-line\ntext.");
+line_index.line_col(3.into()); // LineCol { line: 0, col: 3 }
+line_index.line_col(13.into()); // LineCol { line: 1, col: 3 }
+line_index.offset(LineCol { line: 2, col: 3 }); // Some (24)
+```
+
+## SemVer
+
+This crate uses [semver](https://semver.org/) versioning.