[PATCH] doc: add missing package dependency in contributors guide

[Rami Rosen <ramirose@gmail.com>]

There is a missing depenency for building docs with "make doc-guides-pdf".
This causes it to break with "make[3]: latexmk: Command not found". This 
was observed and reproted in https://bugs.dpdk.org/show_bug.cgi?id=182
This patch fixes this issue by adding the latexmk package dependency in 
sub-section 4.3.1 of the contributing guide ("Dependencies").

Cc: stable@dpdk.org
Signed-off-by: Rami Rosen <ramirose@gmail.com>
---
 doc/guides/contributing/documentation.rst | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/doc/guides/contributing/documentation.rst b/doc/guides/contributing/documentation.rst
index c28a95c34..83e39d189 100644
--- a/doc/guides/contributing/documentation.rst
+++ b/doc/guides/contributing/documentation.rst
@@ -204,6 +204,17 @@ The main required packages can be installed as follows:
    # Red Hat/Fedora, selective install.
    sudo dnf     -y install texlive-collection-latexextra
 
+`Latexmk <http://personal.psu.edu/jcc8/software/latexmk-jcc/>`_ is a perl script for running LaTeX for
+resolving cross references, and it also runs auxiliary programs like bibtex, makeindex
+if necessary, and dvips. It has also a number of other useful capabilities (see man 1 latexmk).
+
+.. code-block:: console
+
+   # Ubuntu/Debian.
+   sudo apt-get -y install latexmk
+
+   # Red Hat/Fedora, selective install.
+   sudo dnf     -y install latexmk
 
 Build commands
 ~~~~~~~~~~~~~~
-- 
2.19.2