readme: add build instructions

Author: chris-martin

README.md

@@ -1 +1,17 @@
-# joyofhaskell.com
+# joyofhaskell.com
+
+## How to build
+
+You will need Stack (and some other things, but they are taken care of for you
+  if you have Nix integration enabled; see `shell.nix`).
+
+First update the git submodules:
+
+```bash
+git submodule init
+git submodule update
+```
+
+Then run `make` to build the site.
+
+See the `Makefile` for other things you can do.