Add zuul.project.src_dir variable

For ease of use in writing ad-hoc jobs, include a variable for the
directory on the nodes where the source code will be.

Change-Id: Idb7b3a14766497d7013e054fbe1279cc8809fdb9
diff --git a/doc/source/user/jobs.rst b/doc/source/user/jobs.rst
index 7f1c3cb..46b36f5 100644
--- a/doc/source/user/jobs.rst
+++ b/doc/source/user/jobs.rst
@@ -202,6 +202,13 @@
          The full canonical name of the project including hostname.
          E.g., `git.example.com/org/project`.
 
+      .. var:: src_dir
+
+         The path to the source code on the remote host, relative
+         to the home dir of the remote user.
+         E.g., `src/git.example.com/org/project`.
+
+
    .. var:: tenant
 
       The name of the current Zuul tenant.
@@ -243,6 +250,12 @@
             The full canonical name of the project including hostname.
             E.g., `git.example.com/org/project`.
 
+         .. var:: src_dir
+
+            The path to the source code on the remote host, relative
+            to the home dir of the remote user.
+            E.g., `src/git.example.com/org/project`.
+
       .. var:: branch
 
          The target branch of the change (without the `refs/heads/` prefix).