Add jaxb-api as external dependency

Compilation is broken because it cannot find classes like
javax.xml.bind.XmlAttribute (JAXB). It seems JAXB was removed in java
11. Add it as external dependency.

This makes the plugin compile. Unit tests seems to be broken, but that
is a different issue.

Change-Id: Id3980064461b1e789967a148fc5c6f52db070f2f
diff --git a/BUILD b/BUILD
index 03fd510..f304e7b 100644
--- a/BUILD
+++ b/BUILD
@@ -11,6 +11,9 @@
         "Implementation-URL: https://gerrit-review.googlesource.com/#/todo",
     ],
     resources = glob(["java/Documentation/**/*.md"]),
+    deps = [
+        "@jaxb-api//jar",
+    ],
 )
 
 junit_tests(
diff --git a/README.md b/README.md
index 803c854..85f48ae 100644
--- a/README.md
+++ b/README.md
@@ -2,4 +2,4 @@
 The plugin will update a superproject to contain the submodules defined in the
 manifest XML file whenever the manifest repo is changed.
 
-For more information see [the documentation](java/Documentation/about.md).
+For more information see [the documentation](java/Documentation/about.md) or the [build instructions](java/Documentation/build.md)
diff --git a/external_plugin_deps.bzl b/external_plugin_deps.bzl
new file mode 100644
index 0000000..81cac86
--- /dev/null
+++ b/external_plugin_deps.bzl
@@ -0,0 +1,7 @@
+load("//tools/bzl:maven_jar.bzl", "maven_jar")
+
+def external_plugin_deps():
+  maven_jar(
+    name = 'jaxb-api',
+    artifact = 'javax.xml.bind:jaxb-api:2.3.1',
+  )
diff --git a/java/Documentation/build.md b/java/Documentation/build.md
new file mode 100644
index 0000000..a447703
--- /dev/null
+++ b/java/Documentation/build.md
@@ -0,0 +1,26 @@
+BUILD
+=====
+
+This plugin must be built inside gerrit source tree with bazel.
+
+1. Checkout gerrit and clone this plugin inside gerrit/plugins directory:
+
+```bash
+$ git clone https://gerrit.googlesource.com/gerrit
+$ cd gerrit
+$ git clone https://....  plugins/supermanifest
+```
+
+2. This plugin has external dependencies. We need to tell gerrit 
+   about it overwriting the external_plugins_deps.bzl:
+
+```bash
+rm plugins/external_plugins_deps.bzl
+ln -s  plugins/supermanifest/external_plugins_deps.bzl plugins/external_plugins_deps.bzl
+```
+
+3. Now it should build with
+
+```bash
+$ bazel build //plugins/supermanifest:supermanifest
+```