Documented authentication to GitHub Packages (#302)

To download the artifact.

README.md

@@ -6,6 +6,9 @@
 
 Maven plugin metadata extractor for plugins written in Kotlin.
 
+**Note:** this JAR isn't available from Maven Central, see below for how to
+configure a `pluginRepository` to download the JAR from GitHub Packages.
+
 **This project is still a little new, so please help by reporting any issues you find.**
 
 ## Maven plugin support for Kotlin
@@ -204,7 +207,8 @@ Your POM will include the following:
   -->
   <pluginRepositories>
       <pluginRepository>
-          <id>kotlin-maven-plugin-tools</id>
+          <!-- id must match the id of the server in settings.xml -->
+          <id>github</id>
           <name>GitHub Packages</name>
           <url>https://maven.pkg.github.com/gantsign/kotlin-maven-plugin-tools</url>
           <snapshots>
@@ -216,6 +220,40 @@ Your POM will include the following:
 </project>
 ```
 
+GitHub Packages repositories require authentication, so you need to specify the
+credentials in your `settings.xml`. You don't need particular permissions, any
+valid GitHub account token will do.
+
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<settings xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xmlns="http://maven.apache.org/SETTINGS/1.0.0"
+    xsi:schemaLocation="http://maven.apache.org/SETTINGS/1.0.0
+                          https://maven.apache.org/xsd/settings-1.0.0.xsd">
+  <servers>
+    <server>
+      <!-- This is setup for you if using id "github" with
+           https://github.com/actions/setup-java -->
+      <!-- id must match the id of the pluginRepository -->
+      <id>github</id>
+      <username>${env.GITHUB_ACTOR}</username>
+      <password>${env.GITHUB_TOKEN}</password>
+    </server>
+  </servers>
+
+</settings>
+```
+
+If using GitHub Actions, you also need to pass the `GITHUB_TOKEN` environment
+variable to the build step(s) in your workflow file e.g.:
+
+```yaml
+- name: Build with Maven
+  run: mvn install
+  env:
+    GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+```
+
 License
 -------