Add commands to build docs PDF to developer HOWTO

docs/development/howto.rst

@@ -507,6 +507,28 @@ We have changed the ``None`` option of `sklearn.utils.check_random_state` to ``'
 because we felt that this meaning for ``None`` was confusing given that `numpy.random.RandomState`
 uses a different meaning (for which we use the option ``'global-rng'``).
 
+Sphinx docs build
+-----------------
+
+Generating the HTML docs for Gammapy is straight-forward::
+
+    python setup.py build_docs
+    open docs/_build/html/index.html
+
+Generating the PDF docs is more complex.
+This should work::
+
+    python setup.py build_docs -b latex
+    cd docs/_build/latex
+    makeindex -s python.ist gammapy.idx
+    pdflatex -interaction=nonstopmode gammapy.tex
+    open gammapy.pdf
+
+You need a bunch or LaTeX stuff, specifically ``texlive-fonts-extra`` is needed.
+
+The PDF is also generated on Read the Docs and available online here:
+https://media.readthedocs.org/pdf/gammapy/latest/gammapy.pdf
+
 Documentation guidelines
 ------------------------
 
@@ -530,6 +552,7 @@ or search the Numpy or Astropy documentation guidelines mentioned above.
 If that doesn't quickly turn up something useful, please ask by putting a comment on the issue or
 pull request you're working on on Github, or send an email to the Gammapy mailing list.
 
+
 Functions or class methods that return a single object
 ++++++++++++++++++++++++++++++++++++++++++++++++++++++