Add usage instructions in the readme.

Author: sjrd

README.md

@@ -5,6 +5,18 @@
 It assumes that the target platform supports JavaScript's `WeakRef` and `FinalizationRegistry`, which are becoming standard in ECMAScript 2021.
 Attempts to use `WeakReference` or `ReferenceQueue` will throw `ReferenceError`s if they are not supported.
 
+## Usage
+
+Add the following dependency to your project settings:
+
+```scala
+libraryDependencies += "org.scala-js" %%% "scalajs-weakreferences" % "1.0.0-SNAPSHOT"
+```
+
+When using a `crossProject`, add the above in `.jsSettings(...)`.
+
+You can then use `java.lang.ref.WeakReference` and `java.lang.ref.ReferenceQueue`.
+
 ## License
 
 `scalajs-weakreferences` is distributed under the [Apache 2.0 license](./LICENSE.txt), like Scala.js itself.