Initial releases.csv for web installations

This is a file for tracking information about Elixir releases, which encodes as `Version,URL,Prerelease,WindowsInstallType`

* `Version` is the Elixir version
* `URL` is the Precompiled.zip corresponding with that version
* `Prerelease` is true or false
* `WindowsInstallType` indexes the type of [Windows installer](https://github.com/chyndman/elixir-windows-setup) that can install this version.

(I elected to put the `WindowsInstallType` last rather than first since any other consumer of this data probably won't care for it.)