The readme (#45)

* The readme

A temporary README to plug the gap. Clearly doesn't resolve #16 .

* Update README.md

More or less Eric's suggestions, but added `(for example)` to indicate that linear algebra + higher order functions over higher-order derivatives is not the only decision which has been motivated by the ML goal (I don't want to give the impression that this is the only thing we've done as a consequence of the ML priority)

README.md

@@ -5,3 +5,11 @@
 [![codecov.io](http://codecov.io/github/invenia/Nabla.jl/coverage.svg?branch=master)](http://codecov.io/github/invenia/Nabla.jl?branch=master)
 [![Stable Docs](https://img.shields.io/badge/docs-stable-blue.svg)](https://invenia.github.io/Nabla.jl/stable)
 [![Latest Docs](https://img.shields.io/badge/docs-latest-blue.svg)](https://invenia.github.io/Nabla.jl/latest)
+
+Nabla.jl is a reverse-mode automatic differentiation package targetting machine learning use cases. 
+As such, we have (for example) prioritised support for linear algebra optimisations and higher-order functions over the ability to take higher-order derivatives (Nabla currently only supports first-order derivatives).
+
+Nabla is currently under active development and improved documentation will be made available soon. 
+Until then, the code in the [examples](src/examples) folder best indicates how to use the package. 
+Given the early stage of development, we anticipate a number of bugs and performance issues. 
+If you encounter any of these or have any particular feature requests, please raise an issue and let us know.