Mention Sphinx dependency for building the docs

CONTRIBUTING.rst

@@ -76,9 +76,10 @@ has a tendency to get outdated. If you encounter outdated pieces of
 documentation feel free to submit a pull request or open a ticket
 pointing out what should be changed or even suggesting a better text.
 
-To get the docs updated the docs need to be build with *make html* in
+To check your changes docs need to be build with *make html* in
 *documentation/src*. This places the compliled documentation in
-*documentation/build/html*.
+*documentation/build/html*. You need to have *sphinx* installed for
+this to work.
 
 The online documentation gets build and updated automatically by the Travis CI
 as soon as the changes makes it into the GitHub master branch.

documentation/src/install.rst

@@ -48,6 +48,14 @@ Boxes.py - while defaulting to Python 3 - can also run on Python 2.7.
 If you encounter any compatibility issues please report them at the
 `GitHub project <https://github.com/florianfesti/boxes>`__.
 
+Sphinx
+......
+
+For building the documentation locally you need the *Sphinx* documentation
+generator (package name may be python-sphinx or python3-sphinx). It is
+not needed for anything else. Boxes.py can be run and changed just
+fine without.
+
 Running from working dir
 ------------------------