Lack of build instructions

[fengweichangzi]

Very nice project about Liegroup.

When I tried to build Sophus, I could not find some instructions E.g., 3rd dependent library like Eigen, or is fmt library necessary and so on.

Maybe adding some build instrucions will reduce some build issues, in my opinion.

[strasdat]

Will add build instructions. Hoping to do so as part of v20.22 release.