Add stack.yaml, update README accordingly

Building proved to be more difficult than it should be, hopefully
stack's reproducible builds will finally be reproducibly successful...

README.md

@@ -2,9 +2,12 @@
 
 Can either be done with [stack][1] or [cabal][2], the easier of which is stack:
 
-    stack init
     stack build
     
+That may ask you to install a compiler. You can either let stack
+install it locally via `stack setup` or use a globally installed one
+with `stack build --system-ghc`.
+
 Optionally, tests can be executed with:
 
     stack test

stack.yaml

+flags: {}
+extra-package-dbs: []
+packages:
+- '.'
+extra-deps:
+- aeson-1.0.2.1
+resolver: lts-7.13