Add purenix backend (#409)

* Add purenix backend

* Update ecosystem/Alternate-backends.md

Co-authored-by: Dennis Gosnell <cdep.illabout@gmail.com>

Co-authored-by: Thomas Honeyman <admin@thomashoneyman.com>
Co-authored-by: Dennis Gosnell <cdep.illabout@gmail.com>

Author: 3 people

ecosystem/Alternate-backends.md

@@ -6,6 +6,7 @@ There are various alternatives to `psc`'s default JavaScript backend:
 |:--------------------------------------------------------------------------------------|:-----------|:--------------|:------|:-----------------------|
 | [andyarvanitis/purescript-native](https://github.com/andyarvanitis/purescript-native)             | 0.13.x    | C++11 or Go         | active | all applicable tests in `purescript/tests/purs/passing` are passing |
 | [purerl/purerl](https://github.com/purerl/purerl) | 0.13.5 | Erlang | active |
+| [purenix-org/purenix](https://github.com/purenix-org/purenix) | 0.14.4 | Nix | active | |
 
 ## Alpha Quality