Update README.

Author: dvgica

README.md

@@ -1 +1,77 @@
-# periodic
+# Periodic
+[![Maven](https://img.shields.io/maven-central/v/ca.dvgi/periodic_2.13?color=blue)](https://search.maven.org/search?q=g:ca.dvgi%20periodic) [![CI](https://img.shields.io/github/actions/workflow/status/dvgica/periodic/ci.yml?branch=main)](https://github.com/dvgica/periodic/actions)
+
+Periodic is a Scala library providing a variable that self-updates on a periodic basis.
+
+- [Motivation](#motivation)
+- [Installation](#installation)
+- [Usage](#usage)
+- [Contributing](#contributing)
+
+## Motivation
+
+This library is useful for caching semi-static data in memory and having that data automatically updated periodically. The source of the data is typically another process that can be queried to get a new data value. If the cached data becomes stale at a predictable interval, the cached data can be updated before this occurs. If the cached data becomes stale at unpredictable times, the stale data must still be usable. Concrete use cases include:
+
+- caching a time-limited key or token, and replacing it with a new one before it expires (e.g. an OAuth access token)
+- caching data that changes irregularly and occasionally, such as a list of a country's airports and their codes
+
+## Installation
+
+Periodic is available on Maven Central for Scala 2.12, 2.13, and 3. Java 11+ is required.
+
+There is currently a `periodic-api` project which exposes an interface for `AutoUpdatingVar`, and a `periodic-jdk` project which implements the API using primitives included in the JDK. Other implementations may follow.
+
+If a specific implementation is required, add the following dependency to your project:
+
+`"ca.dvgi" %% "periodic-jdk" % "<latest>"`
+
+If only the interface is required, add:
+
+`"ca.dvgi" %% "periodic-api" % "<latest>"`
+
+## Usage
+
+``` scala
+import ca.dvgi.periodic.jdk._
+import ca.dvgi.periodic._
+import scala.concurrent.duration._
+import scala.concurrent.Await
+import java.time.Instant
+
+def updateData(): String = Instant.now.toString
+
+val data = new JdkAutoUpdatingVar(
+  updateData(),
+  // can also be dynamic based on the last data
+  UpdateInterval.Static(1.second), 
+  // can also be finite with configurable behavior for attempt exhaustion
+  UpdateAttemptStrategy.Infinite(5.seconds), 
+)
+
+// `ready` returns a `Future[Unit]` which completes when the initial data initialization is complete
+// see also the `blockUntilReadyTimeout` parameter
+Await.result(data.ready, 5.seconds)
+
+// the data is cached in-memory and can be access using `latest`
+println(s"Cached data is ${data.latest}")
+// data has not been updated yet, same result
+println(s"Cached data is still ${data.latest}")
+
+Thread.sleep(2000)
+
+// the `AutoUpdatingVar` fetched new data while this thread was sleeping
+println(s"New cached data is ${data.latest}")
+```
+
+This returns something like:
+```
+Cached data is 2023-10-19T02:35:22.467418Z
+Cached data is still 2023-10-19T02:35:22.467418Z
+New cached data is 2023-10-19T02:35:23.474155Z
+```
+
+For handling errors during update, and other options, see the Scaladocs. 
+
+## Contributing 
+
+Contributions in the form of Issues and PRs are welcome.