Add explanation/examples on how to use utils.priors

[ziatdinovmax]

The utils.priors (code) have beed introduced to streamline the placement of priors over model parameters as well as to simplify the incorporation of prior mean functions. Several examples (in docstrings and/or in a separate notebook) of their usage would be beneficial.

[SandhyaGiribabu]

Hey, I have created a pull request to resolve this issue.
This is my first contribution and would love to hear back from you.