Add vignette of methodology/package

[nikolase90]

I made a template with https://github.com/NorskRegnesentral/shapr/tree/nikolai/add_vignette. You could continue pushing to this branch once you have the time work on this.

Some examples of vignettes:

• https://github.com/thomasp85/lime
• https://github.com/osorensen/BayesMallows

[martinju]

List of features for the vignette

• [x] Short intro with some info on Shapley values for prediction explanation, mentioning that we account for dependence which the original kernelSHAP implementation doesn't
• [x] Overview of main functions
• [x] "Technical" description of the various approximation functions (copy some content from paper)
• [x] Compare our version of independence kernelSHAP with the original implementation checking that we get (essentially) the same results.
• [x] Include readme example of running our method
• [ ] If we have time and bother: An additional, small simulation run with know truth, comparing our methods with independence (and/or TreeSHAP) like in the paper.
• [x] Reference paper for further details and comparisons

[martinju]

Ref #59 I will prepare a code chunk comparing Lundberg's implementation of KernelSHAP with a special case of implementation to check that we get comparable results. Will use example from readme.