[Patch] Please document dia's environment variables

one use case for environment variables for dia: when building dia, a test harness will run. The test harness will use shapes and data from /usr/share/dia, which is incorrect because random shapes installed under /usr/share/dia/shapes (such as those from Debian's additional dia-shapes package) will make the tests fail.

So it's important to having DIA_SHAPE_PATH set before running the test harness. However the existence of DIA_SHAPE_PATH has not been documented until now...