Merge pull request #12 from gradha/pr_cairo_notes

Adds info about libcairo runtime dependency.

README.md

@@ -1,9 +1,23 @@
 nimforum
 ========
 
-This is Nimrod's forum. The code depends on the RST parser of
-the Nimrod compiler and on Jester.
+This is Nimrod's forum. The code depends on the RST parser of the Nimrod
+compiler and on Jester. The code generating captchas for registration uses the
+[cairo module](http://nimrod-code.org/cairo.html), which requires you to have
+the [cairo library](http://cairographics.org) installed when you run the forum,
+or you will be greeted by a cryptic error message similar to:
 
+	$ ./forum could not load: libcairo.so(1.2)
+
+If you are using macosx and have installed the ``cairo`` library through
+[MacPorts](https://www.macports.org) you still need to add the library path to
+your ``LD_LIBRARY_PATH`` environment variable. Example:
+
+	$ LD_LIBRARY_PATH=/opt/local/lib/ ./forum
+
+Replace ``/opt/local/lib`` with the correct path on your system.
+
+# Copyright
 
 Copyright (c) 2012-2013 Andreas Rumpf, Dominik Picheta.