minor: Add settings plugin

To ensure that we reckon can get configured before anyone else will
use the project versions, we're adding a settings plugin. As long as
no one applies another settings plugin that happens to also interact
with versions this should resolve any other issues that woud crop up.

Fixes #173

README.md

@@ -117,11 +117,21 @@ Reckon can alternately use SNAPSHOT versions instead of the stage concept.
 
 #### Apply the plugin
 
+**IMPORTANT:** It is recommended to apply reckon as a Settings plugin (in settings.gradle/settings.gradle.kts) to ensure it is configured before any other plugin tries to use the project version.
+
 ```groovy
+
+// if applying in settings.gradle(.kts)
+plugins {
+  id 'org.ajoberstar.reckon.settings' version '<version>'
+}
+
+// if applying in build.gradle(.kts)
 plugins {
   id 'org.ajoberstar.reckon' version '<version>'
 }
 
+// in either case
 reckon {
   // START As of 0.16.0
   // what stages are allowed

reckon-gradle/build.gradle.kts

@@ -82,5 +82,11 @@ gradlePlugin {
       description = "Infer a project's version from your Git repository."
       implementationClass = "org.ajoberstar.reckon.gradle.ReckonPlugin"
     }
+    create("settings") {
+      id = "org.ajoberstar.reckon.settings"
+      displayName = "Reckon Settings Plugin"
+      description = "Infer a build's version from your Git repository."
+      implementationClass = "org.ajoberstar.reckon.gradle.ReckonSettingsPlugin"
+    }
   }
 }

reckon-gradle/src/compatTest/groovy/org/ajoberstar/reckon/gradle/CompositeBuildCompatTest.groovy

@@ -8,8 +8,6 @@ import org.ajoberstar.grgit.Grgit
 import org.gradle.testkit.runner.GradleRunner
 import org.gradle.testkit.runner.BuildResult
 
-// Composite builds were added in 3.1
-@IgnoreIf({ System.properties['compat.gradle.version'] == '3.0' })
 class CompositeBuildCompatTest extends Specification {
   @TempDir File tempDir
   File project1Dir