Migration documentation

Added a table to help translate old users to new properties.

This fixes #21.

README.md

@@ -75,7 +75,7 @@ gitPublish {
         exclude '1.0.0/temp.txt'
     }
 
-    // message used when committing changes    
+    // message used when committing changes
     commitMessage = 'Publishing a new page' // defaults to 'Generated by gradle-git-publish'
 }
 ```
@@ -102,6 +102,20 @@ jbakeTask {
 gitPublishCommit.dependsOn jbakeTask
 ```
 
+## Migrating from org.ajoberstar.github-pages
+
+The following table should help translate settings you used in `org.ajoberstar.github-pages` to this plugin's format. Additionally reference the Configuration section above for more information on the current feature set.
+
+| org.ajoberstar.github-pages | org.ajoberstar.git-publish | Comment |
+|-----------------------------|----------------------------|---------|
+| `repoUri`                   | `repoUri`                  | Used to allow any Object (which would be lazily unpacked to a String). Now requires a String. |
+| `targetBranch`              | `branch`                   | The old plugin defaulted to `gh-pages`, the new one has no default. This must be a String. |
+| `workingPath`               | `repoDir`                  | Used to allow any Object and called `file()` on it for you. Now expects a File. |
+| `pages`                     | `contents`                 | Just a name change. |
+| `deleteExistingFiles`       | `preserve`                 | If previously `true` (the default), do nothing. If previously `false`, `preserve { include '**/*' }`
+| `commitMessage`             | `commitMessage`            | Just copy from the old value. |
+| `credentials`               | _None_                     | This feature was removed. |
+
 ## Questions, Bugs, and Features
 
 Please use the repo's [issues](https://github.com/ajoberstar/gradle-git-publish/issues)