Document ceres should be built with SuiteSparse support

[v01dXYZ]

Currently, glomap depends explicitly on SuiteSparse.

On my distribution, ceres is not packaged with SuiteSparse support (it only provides the following CMake components: EigenSparse SparseLinearAlgebraLibrary LAPACK SchurSpecializations MultiThreadind).

I suggest to choose between:

• add a note warning about this in README.md.
• simply let ceres pick its sparse linear algebra backend using instead SparseLinearAlgebraLibrary.

Tell me what you think about it and I'll write the small PR if you interested.

[lpanaf]

Hi @v01dXYZ , thanks for pointing this out. A pull request is very much welcomed.