Add a note about realpath to readme

README.md

@@ -9,6 +9,19 @@ may be incomplete)
 See the [Quick Start Guide](./doc/quick-start-guide.md).
 
 
+## Dependencies
+
+This project requires a modern unix system as a base (such as Ubuntu Linux).
+It also requires `bash`, `docker`, and `docker-compose`. 
+
+The `bin/doctor` script can be used to check for missing dependencies.
+
+Note: MacOS does not ship with a `realpath` program. In this case we fall
+back to a custom shell function to imitate some of what `realpath` does, but
+it is relatively limited. We recommend users on MacOS install the gnu coreutils
+with `brew install coreutils`, to get a working version of `realpath`.
+
+
 ## Doctor
 
 Run `bin/doctor` for debug output