Additional documentation for predict_readability

[lcombs]

Within the predict_readability, it could be useful to add a bit more information or reference materials for:

• what lambda represents and its interpretation
• what the default values of the parameters mean for reference_top = -2.1763368548 and
  reference_bottom = -3.865467.

Note: we have some information on where the parameter values come from (fifth grade, bottom and SOTU, top) within the documentation section, but it could be useful to have a description on how increasing or decreasing this value may impact the analysis.