Document `refdoc' usage.

* docs/reference/README: Add information and instructions to use targets
`make refdoc' and the new `make refdoc-venv'. Also add troubleshooting
tips.

docs/reference/README

@@ -1,5 +1,35 @@
-After saying `make refdoc' this directory contains the FreeType API
-reference.  You need python to make this target.
+After saying `make refdoc' or `make refdoc-venv' the `site/' directory
+contains the FreeType API reference.  You need python and pip to make this
+target.
+
+There are two ways to generate the documentation:
+
+1. Using `make refdoc':
+
+    - Ensure `python' and `pip' are available.
+    - Install pip package `docwriter' with `pip install --user docwriter'.
+    - Make target with `make refdoc'.
+    - This target can be run offline once required packages are installed.
+
+2. Using `make refdoc-venv' (requires internet access):
+
+    - Ensure `python', `pip' and python package `virtualenv' are available.
+    - Make target with `make refdoc-venv'.
+    - This may or may not require internet access every time depending on
+    pip and system caching.
 
 This also works with Jam: Just type `jam refdoc' in the main directory.
 
+Some troubleshooting tips:
+
+* Regularly run `pip install --upgrade docwriter' to check for updates which
+may include bug fixes.
+
+* Ensure that `docwriter' is installed in the same python target that
+`make refdoc' uses (python3/python2/python).
+
+* `pip' and `python' may point to different versions of Python.  Check using
+`python --version' and `pip --version'.
+
+* If none of this works, send a mail to `freetype-devel@nongnu.org' or file
+an issue at `https://github.com/freetype/docwriter/issues'.