Add version ranges readme (#266)

* Add version ranges readme

Mostly copied and edited from the crate description.

The image came out underwhelming, but i want to include that visualization as a number line, it's my most used tool when interacting with ranges.

* Better image

* Shorten arrows

version-ranges/Cargo.toml

@@ -1,11 +1,12 @@
 [package]
 name = "version-ranges"
-version = "0.1.0"
+version = "0.1.1"
 description = "Performance-optimized type for generic version ranges and operations on them."
 edition = "2021"
 repository = "https://github.com/pubgrub-rs/pubgrub"
 license = "MPL-2.0"
 keywords = ["version", "pubgrub", "selector", "ranges"]
+include = ["ranges.png", "src"]
 
 [dependencies]
 proptest = { version = "1.5.0", optional = true }

version-ranges/README.md

@@ -0,0 +1,44 @@
+# Ranges
+
+[![crates.io](https://img.shields.io/crates/v/version-ranges.svg?logo=rust)](https://crates.io/crates/version-ranges)
+[![docs.rs](https://img.shields.io/badge/docs.rs-version-ranges)](https://docs.rs/version-ranges)
+
+This crate contains a performance-optimized type for generic version ranges and operations on them.
+
+`Ranges` can represent version selectors such as `(>=1.5.1, <2) OR (==3.1) OR (>4)`. Internally, it is an ordered list
+of contiguous intervals (segments) with inclusive, exclusive or open-ended ends, similar to a
+`Vec<(Bound<T>, Bound<T>)>`.
+
+You can construct a basic range from one of the following build blocks. All other ranges are concatenation, union, and
+complement of these basic ranges.
+
+- `Ranges::empty()`: No version
+- `Ranges::full()`: All versions
+- `Ranges::singleton(v)`: Only the version v exactly
+- `Ranges::higher_than(v)`: All versions `v <= versions`
+- `Ranges::strictly_higher_than(v)`: All versions `v < versions`
+- `Ranges::lower_than(v)`: All versions `versions <= v`
+- `Ranges::strictly_lower_than(v)`: All versions `versions < v`
+- `Ranges::between(v1, v2)`: All versions `v1 <= versions < v2`
+
+The optimized operations include `complement`, `contains`, `contains_many`, `intersection`, `is_disjoint`,
+`subset_of` and `union`.
+
+`Ranges` is generic over any type that implements `Ord` + `Clone` and can represent all kinds of slices with ordered
+coordinates, not just version ranges. While built as a performance-critical piece
+of [pubgrub](https://github.com/pubgrub-rs/pubgrub), it can be adopted for other domains, too.
+
+![A number line and a sample range on it](number-line-ranges.svg)
+
+You can imagine a `Ranges` as slices over a number line.
+
+Note that there are limitations to the equality implementation: Given a `Ranges<u32>`, the segments
+`(Unbounded, Included(42u32))` and `(Included(0), Included(42u32))` as well as
+`(Included(1), Included(5))` and  `(Included(1), Included(3)) + (Included(4), Included(5))`
+are reported as unequal, even though the match the same versions: We can't tell that there isn't a version between `0`
+and `-inf` or `3` and `4` respectively.
+
+## Optional features
+
+* `serde`: serialization and deserialization for the version range, given that the version type also supports it.
+* `proptest`: Exports are proptest strategy for `Ranges<u32>`.