Add in-tree buck build

Change-Id: I308da1cdab46febbc0edadd76a00adaaf9c83854

BUCK

diff --git a/BUCK b/BUCK
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--- /dev/null
+++ b/BUCK
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+gerrit_plugin(
+  name = 'menuextender',
+  srcs = glob(['src/main/java/**/*.java']),
+  resources = glob(['src/main/resources/**/*']),
+  manifest_entries = [
+    'Gerrit-PluginName: menuextender',
+    'Gerrit-Module: com.googlesource.gerrit.plugins.menuextender.Module',
+    'Gerrit-HttpModule: com.googlesource.gerrit.plugins.menuextender.HttpModule',
+  ]
+)

src/main/resources/Documentation/build.md

diff --git a/src/main/resources/Documentation/build.md b/src/main/resources/Documentation/build.md
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index 0000000..76890aa
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+++ b/src/main/resources/Documentation/build.md
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+Build
+=====
+
+This plugin is built with Buck.
+Clone or link this plugin to the plugins directory of Gerrit tree
+and issue the command:
+
+```
+  buck build plugins/menuextender
+```
+
+The output is created in
+
+```
+  buck-out/gen/plugins/menuextender/menuextender.jar
+```
+
+This project can be imported into the Eclipse IDE:
+
+```
+  ./tools/eclipse/project.py
+```
+
+Note for compatibility reasons Maven build is provided, but it considered to
+be deprecated and is going to be removed in one of the future versions of this
+plugin.
+
+```
+  mvn clean package
+```
+
+When building with Maven, the Gerrit Plugin API must be available.
+How to build the Gerrit Plugin API is described in the [Gerrit
+documentation](../../../Documentation/dev-buck.html#_extension_and_plugin_api_jar_files).