start on affine docs

Author: cscherrer

docs/src/affine.md

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+# Affine Transformations
+
+It's very common for measures to be parameterized by `μ` and `σ`, for example as in `Normal(μ=3,σ=4)` or `StudentT(ν=1,μ=3,σ=4)`. In this context, `μ` and `σ` do not always refer to the mean and standard deviation (the `StudentT` above is equivalent to a Cauchy, so both are undefined).
+
+Rather, `μ` is a "location parameter", and `σ` is a "scale parameter". Together, these determine a transform 
+
+```math
+x → σx + μ
+```
+