Update priors documentation

[wwiecek]

Three docs contain info about prior:

1. ?baggr
2. ?normal etc. (a single file with prior definitions)
3. baggr.Rmd with the following text:

As discussed above, it is possible for the user to override the default priors and specify her own priors. The first parameter you can change is the upper bound of the uniform prior on $\sigma_{\tau}$, called "prior_upper_sigma_tau", the second is the mean of the prior on $\tau$ itself called "prior_tau_mean", the third is the scale (standard deviation) of the prior on $\tau$ itself called "prior_tau_scale". Here I have changed all three as an example (at this stage in baggr if you want to alter one you must provide the full vector):

baggr(schools, "rubin", prior = c("prior_upper_sigma_tau" = 10000, 
                                  "prior_tau_mean" = -10, 
                                  "prior_tau_scale" = 100))
baggr_schools