- add PoolSingleton to __init__

- update readme

README.md

@@ -12,8 +12,13 @@ Tested on linux, Windows and OSX with python 3.10 and 3.12.
 - Easy to use
 - Using dill instead of pickle: a lot more objects can be used when parallelizing
 - Progress bars are built-in
+- Automatically use multithreading instead of multiprocessing when the GIL is disabled
 
 ## How it works
+This depends on whether the GIL is currently disabled or not. Disabling the GIL in Python is currently an experimental
+feature in Python3.13, and not the standard.
+
+### Python with GIL enabled
 The work you want parfor to do is divided over a number of processes. These processes are started by parfor and put
 together in a pool. This pool is reused when you want parfor to do more work, or shut down when no new work arrives
 within 10 minutes.
@@ -24,6 +29,12 @@ worker is requesting it. The manager deletes objects automatically when they're
 
 When the work is done the result is sent back for collection in the main process.
 
+### Python with GIL disabled
+The work you want parfor to do is given to a new thread. These threads are started by parfor and put together in a pool.
+The threads and pool are not reused and closed automatically when done.
+
+When the work is done a message is sent to the main thread to update the status of the pool.
+
 ## Installation
 `pip install parfor`
 
@@ -35,12 +46,14 @@ an iterator.
 tqdm, dill
 
 ## Limitations
-Objects passed to the pool need to be dillable (dill needs to serialize them). Generators and SwigPyObjects are examples
-of objects that cannot be used. They can be used however, for the iterator argument when using parfor, but its
-iterations need to be dillable. You might be able to make objects dillable anyhow using `dill.register` or with
-`__reduce__`, `__getstate__`, etc.
+If you're using Python with the GIL enabaled, then objects passed to the pool need to be dillable (dill needs to
+serialize them). Generators and SwigPyObjects are examples of objects that cannot be used. They can be used however, for
+the iterator argument when using parfor, but its iterations need to be dillable. You might be able to make objects
+dillable anyhow using `dill.register` or with `__reduce__`, `__getstate__`, etc.
 
 ## Arguments
+To functions `parfor.parfor`, `parfor.pmap` and `parfor.gmap`.
+
 ### Required:
     fun:      function taking arguments: iteration from  iterable, other arguments defined in args & kwargs
     iterable: iterable or iterator from which an item is given to fun as a first argument