Add disclaimer to README

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

README.md

@@ -4,6 +4,13 @@ Market simulator.
 
 Details coming soon...
 
+# Disclaimer
+
+This is currently in-development. I'm actively deleting migrations, because I want to avoid a lot of
+migrations as I feel out what the models should look like for the initial release. Until this
+software reaches a point release, don't expect your database to be compatible from one commit to the
+next.
+
 # Running
 
 1. Install pipenv with `pip3 install --user pipenv` (make sure `$HOME/.local/bin` is in your `PATH`)