Add gsl to README prerequisites for building from source

[zhuber21]

I was trying to build so3g from source today on a fresh install, and the make step failed because I did not have gsl installed. It seems that gsl was added as a requirement with this commit (78ef713). The documentation about building from source should be updated to include this requirement. As far as I can tell, this just requires adding the "libgsl-dev" to the Linux prerequisites and "gsl" to the homebrew ones.

I tried to make this change on a branch, but it appears that I don't have write access to the repo.

[tskisner]

I need to make some updates to the docs in #192, as part of the build system updates, so I can add gsl to the dependencies mentioned there.