Update README.md

[Davidbrcz]

Add dependency about GLU-dev.

Without that package on my system, a stack build was failing without any kind of obvious error, and one had to go though pages of stack build --verbose to find ... a warning about it.

[jonascarpay]

Hey, thanks for the PR, sorry to hear you had trouble building. I'm hesitant to merge it though; afaict, glu is a transitive dependency of gloss, which would means this only applies to apecs-gloss. We could add a note to apecs-gloss, but gloss requires many more libraries thatn just glu, and I don't think it makes sense to list all of them.

[dpwiz]

I'd say "For examples to be built ..."