add instructions on how to generate grayscale images

README.rst

@@ -32,6 +32,14 @@ The thesis can be built by just saying::
 
     make
 
+To make with grayscale images, say::
+
+    make GRAYSCALE_IMAGES=1
+
+If your machine has multiple cores (e.g 4) you can pass the arguments `-j 4`
+to `make` to do a parallel build. This is especially useful for converting
+the images.
+
 I use `XeTeX <https://en.wikipedia.org/wiki/XeTeX>`_, and hence require
 the `xelatex` program to compile the thesis. In addition a number of
 LaTeX packages are required. On Debian-derived systems it should be