Bazel: Add standalone build mode

Test Plan:

1. Build the plugins:

  $ bazel build :all
  INFO: Analysed target //:all (26 packages loaded).
  INFO: Found 1 target...
  Target //:all up-to-date:
    bazel-genfiles/all.zip
  INFO: Elapsed time: 1.060s, Critical Path: 0.41s

2. Verify the content is sane:

  $ unzip -t bazel-genfiles/all.zip
  Archive:  bazel-genfiles/all.zip
      testing: owners.jar               OK
      testing: owners-autoassign.jar    OK

3. Run the tests:

  $ bazel test //...
  INFO: Found 1 test target...
  Target //owners-common:test up-to-date:
    bazel-bin/owners-common/test.jar
    bazel-bin/owners-common/test
  INFO: Elapsed time: 4.941s, Critical Path: 3.18s
  INFO: Build completed successfully, 13 total actions
  //owners-common:test                           PASSED in 1.9s

4. Generate Eclipse project:

   $ tools/eclipse/project.sh
   Verify project can be imported to Eclipse and dependencies are in the
   classpath

Change-Id: I211bd5acb293a36f9c12b428fbb0a4ceb4dfc177
diff --git a/README.md b/README.md
index 72adf36..aee2d8e 100644
--- a/README.md
+++ b/README.md
@@ -39,8 +39,46 @@
 
 ## How to build
 
-Create three symbolic links of the owners-owners, owners-common and owners-autoassign
-from the Gerrit source code /plugins directory to the subdirectories of this project.
+This plugin is built with Bazel and two build modes are supported:
+
+ * Standalone
+ * In Gerrit tree
+
+### Build standalone
+
+To build the plugin, issue the following command:
+
+```
+  bazel build :all
+```
+
+The output is created in
+
+```
+  bazel-genfiles/owners/owners.jar
+  bazel-genfiles/owners-autoassign/owners-autoassign.jar
+
+```
+
+To execute the tests run:
+
+```
+  bazel test //...
+```
+
+This project can be imported into the Eclipse IDE:
+
+```
+  ./tools/eclipse/project.sh
+```
+
+## Build in Gerrit tree
+
+Create symbolic links of the owners and owners-autoassign folders and of the
+external_plugin_deps.bzl file to the Gerrit source code /plugins directory.
+
+Create a symbolic link of the owners-common plugin to the Gerrit source code
+directory.
 
 Then build the owners and owners-autoassign plugins with the usual Gerrit
 plugin compile command.
@@ -63,3 +101,25 @@
 and does not need to be built separately being a direct dependency of the build
 process. Its resulting .jar must not be installed in gerrit plugins directory.
 
+The output is created in
+
+```
+  bazel-genfiles/plugins/owners/owners.jar
+  bazel-genfiles/plugins/owners-autoassign/owners-autoassign.jar
+```
+
+To execute the tests run:
+
+```
+  bazel test owners-common:test
+```
+
+This project can be imported into the Eclipse IDE:
+
+Add the plugin name to the `CUSTOM_PLUGINS` in Gerrit core in
+`tools/bzl/plugins.bzl` file and run:
+
+```
+  ./tools/eclipse/project.py
+```
+