contributing: mention split documentation

CONTRIBUTING.md

@@ -36,7 +36,7 @@ In addition, we add the following guidelines:
* Only ever `import *` to make objects available in another namespace,
  preferably in *\_\_init\_\_.py*. Everywhere else use explicit object
  imports.
* Use [Napoleon]-comaptible (e.g. NumPy style) docstrings, preferably with
* Use [Napoleon]-compatible (e.g. NumPy style) docstrings, preferably with
  [tests].
* When instantiating Qt widgets, pass static property values as
  [keyword args to the constructor] instead of calling separate property
@@ -182,8 +182,10 @@ Read more [about squashing commits].

Documentation
-------------
Documentation in located in doc folder. You can build it with:
Documentation in located in doc folder. It is split into three parts:
data-mining-library (scripting api), development (development guides),
and visual-programming (widget help files). You can build it with:

    cd doc
    cd doc/<part>
    make html
    # Now open build/html/index.html to see it