[readme] Add a note on changing directory to import compiler_gym.
This problem is a pure "gotcha", and error message produced is
confusing and unfriendly. See
@@ -82,6 +82,10 @@ To build and install the python package, run:
$ make install
+**NOTE:** To use the python code that is installed by `make install` you must
+leave the root directory of this repository. Attempting to import `compiler_gym`
+while in the root of this repository will cause import errors.
+
When you are finished, you can deactivate and delete the conda
environment using:
