add some semblance of guide

README.MD

@@ -1,3 +1,21 @@
 # St Steamus Prime
 
 A steam family optimizer
+
+
+## How to develop
+
+Frankly we are not quite sure.
+
+The flake here sets up a development environment including postgres. You can use it with
+`nix develop --impure`, and then running `devenv up`.
+This process manager thing will eat that shell session, so you have to open another one to acutally develop..
+Don't forget to run `nix develop --impure` in that new session as well!
+
+(or use direnv to do it automatically)
+
+After that you can enter the st_steamus_prime subdirectory and run.
+
+* `mix deps.get` to download the dependencies.
+* `mix phx.server` to run the development server
+