NF: add requirements for building docs

Use with:

    pip install -r doc-requirements.txt

Author: matthew-brett

.travis.yml

@@ -118,8 +118,8 @@ script:
               COVER_ARGS="--with-coverage --cover-package nibabel";
           fi
           if [ "$DOC_DOC_TEST" == "1" ]; then
-              pip install sphinx numpydoc texext;
               cd ../doc;
+              pip install -r ../doc-requirements.txt
               make html;
               make doctest;
           else

doc-requirements.txt

@@ -0,0 +1,6 @@
+# Requirements for building docs
+-r requirements.txt
+sphinx
+numpydoc
+texext
+matplotlib>=1.3

doc/README.rst

@@ -0,0 +1,9 @@
+#####################
+Nibabel documentation
+#####################
+
+To build the documentation, change to the root directory (containing
+``setup.py``) and run::
+
+    pip install -r doc-requirements.txt
+    make html