Add some developer notes

Author: ilevkivskyi

README.md

@@ -12,6 +12,18 @@ synced from the `mypyc/lib-rt`
 
 Report any issues in the [mypyc issue tracker](https://github.com/mypyc/mypyc/issues).
 
+## Developer notes
+
+Since this repo should be kept in sync with `mypy`, it has an unusual directory structure.
+If you want to install from sources, it is recommended to use `sdist` wheel. If you want to
+install directly form the repo, you will need to execute `cp -r lib-rt/* .` before install.
+See issue #17 for discussion.
+
+Unfortunatelly PyPy is not supported. If you develop a library that supports PyPy and you need
+`librt` only as a `mypy` dependency, you can skip PyPy in your CI matrix when type checking
+(since results of type checking will be identical on e.g. CPython 3.11 and PyPy 3.11).
+See issue #16 for discussion.
+
 ## Making a release
 
 1. As a prerequisite, there generally will be some changes in the mypy repository under `mypyc/lib-rt`