Add note about how to do clean docs builds.

docs/README.rst

@@ -1,61 +1,69 b''
 IPython Documentation
 ---------------------
 
 This directory contains the majority of the documentation for IPython.
 
 
 Deploy docs
 -----------
 
 Documentation is automatically deployed on ReadTheDocs on every push or merged
 Pull requests.
 
 
 Requirements
 ------------
 
 The documentation must be built using Python 3.
 
 The following tools are needed to build the documentation:
 
  - sphinx
  - sphinx_rtd_theme
 
 On Debian-based systems, you should be able to run::
 
     sudo apt-get install python3-sphinx python3-sphinx-rtd-theme
 
 In a conda environment, you can use::
 
     conda install sphinx sphinx_rtd_theme
 
 In a Python 3 ``venv``, you should be able to run::
 
     pip install -U sphinx sphinx_rtd_theme
 
 
 Build Commands
 --------------
 
 The documentation gets built using ``make``, and comes in several flavors.
 
 ``make html`` - build the API and narrative documentation web pages, this is
 the default ``make`` target, so running just ``make`` is equivalent to ``make
 html``.
 
 ``make html_noapi`` - same as above, but without running the auto-generated API
 docs. When you are working on the narrative documentation, the most time
 consuming portion  of the build process is the processing and rending of the
 API documentation. This build target skips that.
 
 ``make pdf`` will compile a pdf from the documentation.
 
 You can run ``make help`` to see information on all possible make targets.
 
+To save time,
+the make targets above only proceess the files that have been changed since the
+previous docs build.
+To remove the previous docs build you can use ``make clean``.
+You can also combine ``clean`` with other targets,
+for example,
+``make clean html`` will do a complete rebuild of the docs.
+
 
 Continuous Integration
 ----------------------
 
 Documentation builds are included in the Travis-CI continuous integration process,
 so you can see the results of the docs build for any pull request at
 https://travis-ci.org/ipython/ipython/pull_requests.