Add some commands the build documentation

Before removing standalone build mode, some commands existed only under
standalone build mode. This change adds all the commands under the
Gerrit in-tree build mode.

Change-Id: I55d0248f0fd98d17947b50a5a6bda01e9fed66cc

src/main/resources/Documentation/build.md

diff --git a/src/main/resources/Documentation/build.md b/src/main/resources/Documentation/build.md
index 34881f6..9d539db 100644
--- a/src/main/resources/Documentation/build.md
+++ b/src/main/resources/Documentation/build.md
@@ -61,6 +61,18 @@
   bazel-bin/plugins/@PLUGIN@/@PLUGIN@.jar
 ```
 
+To package the plugin sources run:
+
+```
+  bazel build plugins/@PLUGIN@:lib@PLUGIN@__plugin-src.jar
+```
+
+The output is created in:
+
+```
+  bazel-bin/plugins/@PLUGIN@/lib@PLUGIN@__plugin-src.jar
+```
+
 To execute the tests run:
 
 ```