Add contributing guidelines

[GidonFrischkorn]

I saw that GitHub recommends to write contributing guidelines to lay out what is considered a sufficient contribution to be listed as a contributor. I think this would be a good idea, to give potential contributors an idea what would be expected from them to have their contributions visible for the package.

I will just start a list and maybe we can collect contributions that would pass this threshold.

Level of contributions for becoming a contributor:

• Add a new model to bmm including documentation (vignette, parameter recovery, testing)
• Contribute base functionality that eases the use of all models (e.g. plotting functions, data preparation, post-processing of results)
• Contribute functionality that integrates important functionality from related packages (e.g. emmeans, tidybayes, bayesplot, etc.) to bmm

Levels of contributions mentioned in the NEWS

• bug fixes to models or core functionality in bmm
• contributions to improve stability or speed of estimating bmmodels

This is just a start, and please do post suggestions to clarify these contributions.

[venpopov]

Looks good! Great idea to have this for transparency

[GidonFrischkorn]

Perfect, then I can take care of this.