Merge branch 'gm/readme' into 'master'

update README

See merge request glenn.moynihan/models.jl!1

README.md

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 [![Latest](https://img.shields.io/badge/docs-latest-blue.svg)](https://invenia.pages.invenia.ca/Models.jl/)
 [![Build Status](https://gitlab.invenia.ca/invenia/Models.jl/badges/master/build.svg)](https://gitlab.invenia.ca/invenia/Models.jl/commits/master)
 [![Coverage](https://gitlab.invenia.ca/invenia/Models.jl/badges/master/coverage.svg)](https://gitlab.invenia.ca/invenia/Models.jl/commits/master)
+
+This package defines the `Model` type and a common API for constructing a generic model, including
+
+* Model Fitting (`fit`, `predict`)
+* Model Traits (`output_type`, `estimate_type`)
+* Test utils for testing downstream interfaces (`FakeModel`)
+
+For common examples of the interface being implemented see [BaselineModels.jl](https://gitlab.invenia.ca/invenia/research/BaselineModels.jl).