(patch) Update the README

README.md

@@ -17,3 +17,23 @@ externalResolvers +=
 val GsSlug: ModuleID =
   "gs" %% "gs-slug-v0" % "0.1.0"
 ```
+
+## Slug Type
+
+`Slug` is the type exposed by this library. It is an extremely small, 
+restricted, opaque type (`String`) that adheres to the following regular 
+expression:
+
+```
+^[a-z0-9]+(?:\-[a-z0-9]+)*$
+```
+
+This type is intended for use in the following scenarios:
+
+- Restricted ASCII is acceptable.
+- URL safety is desired.
+
+Additionally, `Slug` is usually intended to be unique within some _context_,
+where the implementation defines the context. For example, if some organization
+owns a number of repositories, each repository might have a `Slug` which is
+unique within that organization.