Clarify limitation about single process on windows/mac

README.md

@@ -32,6 +32,20 @@ There are two ways to install this tool:
    - Obviously the user will need the existing Python install, but other dependencies are automatically installed by Pip.
    - Once installed into the Python install, there will be a binary available to run: `eplus_regression_runner`. 
 
+### Limitations
+
+There are a couple limitation "gotchas" in here, however.  A couple statements ahead of this:
+ - When we create the standalone installer, we use `pyinstaller` to _freeze_ the program and all dependencies.
+ - When we run EnergyPlus, we have to run in multiple processes, not just multiple threads, because of thread-unsafety within EnergyPlus itself.
+
+There is an issue with the combination of these two things that cause the program to not work well on Windows and Mac.
+If you try to freeze the program but use the multiprocessing library to create child instances, it fails.
+There are notes on the web about how to remedy this by calling special functions at the entry of the code, but I could not get them to work fully.
+So for now, if you use the frozen downloadable version of the program on Windows or Mac, it will not run EnergyPlus in multiple processes.
+
+However, if you install the library into a Python install using Pip, the program is never frozen using `pyinstaller`, and seems to work just fine across platforms even with multiprocessing.
+The best install path is to run in that fashion, but if you cannot, you can download the frozen version and just accept the single process for now.
+
 ## Development
 
 For setting up a development environment to do _work_ on this tool, the steps are pretty minimal:

epregressions/runtests.py

@@ -354,7 +354,7 @@ def run_build(self, build_tree):
         # `apply_async` approach I am using.  Blech.  Once again, on Windows, this means it will partially not be
         # multithreaded.
         if frozen and system() in ['Windows', 'Darwin']:  # pragma: no cover -- not covering frozen apps in unit tests
-            self.my_print("Ignoring num_threads on frozen Windows instance, just running with one thread.")
+            self.my_print("Ignoring num_threads on frozen Windows/Mac instance, just running with one thread.")
             for run in energy_plus_runs:
                 ep_return = self.ep_wrapper(run)
                 self.ep_done(ep_return)